The objective of this study which was on “The Development of People Sector’s Guidelines & Method of Assessment on the Health Impact from Government’s Large Investment & Development and Policy: Case-study of Assessment on Health Impact from the Pak Moon Dam Hydroelectric Power Plant Project” was to study about the impact on the health of the communities lying along the bank of the Moon River that had been affected by the Pak Moon Dam Hydroelectric Power Plant Project. The data was analyzed and used for the health impact assessment. In this study, the health impact assessment frame was used to build up a public policy on health. Four communities i.e. Ban Hua Weh, Ban Tha Phae and Ban Wang Sabaeng Tai of Khong Jiam district, and Ban Nong Pho of Phibun Mangsahan, Ubon Ratchathani province were selected as sites of the study. The criteria for community-selection were: (1) Location of community: dam-downstream community, headwork community and dam-upstream community, (2) Nature of impacts: community that lost fishing profession only; transferred community that lost both occupational land and fishing profession. Findings of the study: The villagers defined good health as, “a happy living”. By villagers’ definition, 5 factors that determined health were: (1) Natural resource stability, (2) Food stability, (3) Economic stability, (4) Environmental stability, and (5) Social stability. The study discovered that Pak Moon Dam was an important factor for the change of the factor that determined health. This was because natural resources which were the base of some portion of villagers’ production had been destroyed and become deteriorated by the construction of the dam. As a result, the villagers lost food stability, economic stability, Social culture stability and life happiness. But, when a trial was made for the lifting of the Pak Moon Dam water-gate, the factors that determined all aspects of villagers’ health rapidly returned. However, the condition was not as good as when the dam was not yet constructed. As a result of the trial, health, by villagers’ definition, became better than when the water - gate was closed.The study on the assessment of health impacts from Pak Moon Dam hydroelectric power plant found out that the Pak Moon Dam construction had impacts on physical, mental, social and spiritual health of the villagers. Regarding physical health, some sicknesses related to itchiness, respiratory system and stress occurred to the villagers. But during the water-gate lifting period, some sicknesses i.e. itchiness started to disappear. This was because the water from the Moon River had been drained to the Mae Khong River. Besides, concerning physical health, there were some other sicknesses which were of villagers’ concern such as AIDS, diabetes, liver-related sicknesses, stomach-related sicknesses, etc. Regarding mental health, it was found out that the dam construction was a factor for the villagers’ stressfulness. The villagers became worried about livelihood. They were stressed with neighbors & relatives who disagreed with the dam construction. Conflicts occurred within family, community and the public. Due to water-gate lifting, the mental condition of the villagers became, to certain extent, more relaxed. The villagers were able to catch more fish than when the dam was closed. However, they felt not secure yet for investment on fishing or agriculture because they were not sure of the dam-opening period.Regarding social health, it was found out that dam construction was a factor of change. It decreased relationships among family-members because family members had migrated to work outside communities; joint activities decreased. Communities were in a situation where people just lived for themselves because they had to struggle for their own livelihood. As a result, sharing & caring for one another decreased; cooperation in development activities also decreased. Ideas, belief and wisdom on Moon River became less important. But, during the period of water-gate lifting for water to flow, a clear change occurred i.e. communities started to practice Moon River-related activities i.e. “Boon Nao Kaeng” and “Sao Luang Mong” festivals; families & communities together went for fishing.Regarding health in the spiritual aspect, it was found out that the dam was a factor for the change in the way of life that used to be favorable to the spiritual health of the Pak Moon villagers. They became lack of a way of life that relied on nature. Cultural diversity decreased. Generosity, caring and sharing with one another decreased. Good relationships within family and community became lacking. Transference of wisdom from parents to children became lacking. Areas for communal use such as temples and islets in Moon River became lacking. Communal activities such as religious festivals, recreation, community development became lacking. And, serene, peaceful and happy life became lacking. Consequentially, with regard to spiritual health, individuals, families, communities and the public were living in a state of craving. They were shaken & worried amidst conflicts. They did not have peace and live unhappily.This study revealed that the dam was an important factor for a change in the health aspect which represented the villagers’ and communities’ way of life. Therefore, before, during and after implementation, development projects of different aspects need to assess their impacts on health. It is necessary to have a holistic concept and view on health and to foster environmental and social relationships.