This qualitative study is about the Thai primary health system's new mission and its impacts on the Provincial Administrative Organizations (PAO). The Thai primary health system's mission is a legal mechanism and process for the provision of primary health care services under the Thai Primary Health System Act of 2019 (B.E. 2562). It has never appeared in the powers and duties of the PAO before, which leads to obstacles in management that affect the provision of health services to the public. This research had 3 objectives: 1) to study how transferring health missions to the PAO had effects on the health services at both Chaloem Phrakiat Nawaminthachini Health Center (CPNHC) and Sub-district Health Promoting Hospital (SHPH), and 2) to study how the medical staff were taught about that transition, and 3) to develop policy proposals for the primary health service model during the transition period, using qualitative research methods. The results of this qualitative research showed that the primary health service system (PHSS) of CPNHC and SHPH after the transfer to the PAO focused on providing services in 5 dimensions, namely 1) primary medical care, 2) health promotion, 3) treatment, 4) disease prevention and control, and 5) rehabilitation, and consumer protection using the concept of the Primary Care Cluster (PCC) with the seamless services following the rights under the National Health Security Office (NHSO). The PHSS after the transfer to PAOs should be strengthened under the mechanism of Primary Health System Act (PHSA) B.E.2562 (2019) and other laws related to primary health services and Health Professions Act of health care providers at the primary level. There is a governance of the health system that aims to create a relationship between various sectors in providing a network of health services rather than controlling, supervising, monitoring, but emphasizing on the management of policy networks at 4 different levels: 1) the ministry level, 2) the department level, 3) the health service area, provincial, and district level, and 4) the sub-district level, in order to be able to work in an integrated way to meet health needs at the district, provincial, regional, and national levels. Factors affecting the success of administering and managing the PHSS of the CPNHC and SHPH after transferring to the PAO under the PHSA B.E. 2562 consists of strategic implementation, monitoring, evaluation and the encouragement of strategic implementation as well as promoting the linkage of health services in response to health development policies at each level and the overall picture of the country. Key policy recommendation for primary health care (PHC) systems on the development process of policies and strategies in primary care during the transitional period after the transfer of missions as follows. 1) The Ministry of Public Health (MOPH) should be the host for policy-making and strategies for the primary care system. 2) Health Systems Research Institute (HSRI), National Board of Health (NBH), and Thai Health Promotion Foundation (THPF) collaborate in creating and synthesizing knowledge for development of primary health system in accordance with the policy of the Ministry of Public Health and creating a system and mechanism leading to the concrete implementation. 3) Strategic plan unit and primary care unit of Provincial Health Office, and PAO apply the knowledge acquired at the macro level to create a system and mechanism for learning into practice of the CPNHCs and SHPHs and to design a system to supervise and monitor the development of the PHSS continually. 4)Network node hospitals collaborate with family physicians or PHC providers in CPNHC and SHPH. 5) The CPNHCs and the SHPHs either under the jurisdiction of the MOPH or PAO conduct the strategic implementation as cascaded from the Provincial Health Office and PAO and synthesize it into practical guidelines in accordance with the context including designing a mechanism for supervising and monitoring primary health services, 6)The Provincial Public Health Office (PPHO) coordinates policy cooperation and builds a network for cooperation in collecting health indicator data with the PAO to cover the performance, policy of the country and the problems of the area.7) The responsible agencies for health information technology, MOPH analyze and design web application programing in collaboration with other agencies related to data in supporting health system governance. 8) PAO and PPHO make an agreement about an appropriate proportion of OP (basic payment). 9)MOPH set a policy for integration of resources in primary care system management and monitoring PCU and NPCU as well as coordinating the formation of a referral service network with PCU and NPCU in that area or other areas in accordance with the PHS Act B.E. 2562. Phase for strengthening the PHSS consists of 1) The MOPH sets a policy to develop the quality of primary care facilities across the country to meet standards to certify the quality of services delivered to people with equity. 2) The primary health system committee establishes a system and mechanism for developing primary care standards to be in accordance with section 24 (1), (2), (3) and (7). Sustainable development phase includes 1) The Cabinet should appoint a health system governance committee who understands the national health system as an authority to consider, supervise, monitor, inspect and direct health agencies to review the implementation and provision of health services in accordance with national health policies to continually improve the quality of health services and 2) The transferred CPNHC and SHPS under the PAO create a guideline for the participation of network partners in the local health system jointly providing health services and monitoring and evaluating to meet standards. PHS Act B.E. 2562 and in accordance with the guidelines set forth at the provincial and district levels.