In Thailand, the health care expenditure for the Civil Servant Medical Benefit Scheme (CSMBS) shows an upward trends from fiscal year 2006 to 2009, with the growth rates at 1.66%, 18.12% and 25.61%, respectively. Reimbursement data shows that main driver of the growth was expenditure on outpatient care, derived from drugs cost, in which accounted for 80% of the whole expenses on outpatient services. Therefore, the Comptroller General's Department (CGD), which is responsible for managing the scheme, has launched cost containment measures since 2010. This paper aims to understand pattern of service utilization of CSMBS members to better control of its’ cost.