This research aimed to analyze the expenditure in patients with diabetes mellitus. This study was a retrospective descriptive study. Data was collected from electronic medical record by HOSxP of Bangbuathong Hospital in Nonthaburi since 1 January 2012 to 31 December 2014. Cost analysis was based on 2 concepts of disease classification. The concept 1 was chronic disease group system and the concept 2 was complication risk assessment system. Descriptive statistics such as frequency, percentage, mean, was performed for baseline characteristics, and analytical statistics such as coefficient of variation (CV) and reduction in variance (RIV) for testing of classification. The results showed that there were 1,066 patients with diabetes mellitus. Most of them were women (71.95%). The average age was 65.42 ± 10.29 years and most were covered by the universal health coverage scheme (86.87%). The average expenditure was 6,442.26 baht per person per year (SD = 3,904.78). Disease classification by both concepts had CV lower than 150% and decrease RIV. The concept of chronic disease group system had clarity in the pattern of payment, pattern of disease and items of medication. The concept of complication risk assessment system had clarity only on the pattern of payment, but not on the pattern of disease or items of medication. Further research and development on outpatient disease classification should focus on the concept of chronic disease group system.