Purpose: This study was to ascertain whether there are differences in health care utilization and expenditure for Type I Medical Aid Beneficiaries before and after applying Copayment. Methods: This study was one-group pretest posttest design study using secondary data analysis. Data for pretest group were collected from claims data of the Korea National Health Insurance Corporation and data for posttest group were collected through door to-door interviews using a structured questionnaire. A total of 1,364 subjects were sampled systematically from medical aid beneficiaries who had applied for copayment during the period from December 12, 2007 to September 25, 2008. Results: There was no negative effect of copayment on accessibility to medical services, medication adherence (p=.94), and quality of life (p=.25). Some of the subjects` health behaviors even increased preferably after applying for copayment including flu prevention (p<.001), health care examination (p=.035), and cancer screening (p=.002). However, significant suppressive effects of copayment were found on outpatient hospital visiting days (p<.001) and outpatient medical expenditure (p<.001). Conclusion: Copayment does not seem to be a great influencing factor on beneficiaries` accessibility to medical services and their health behavior even though it has suppressive effects on outpatients` use of health care.