Objectives : It is necessary to monitor consumption of drugs in order to enhance promote appropriate use of drugs. Defined Daily Dose(DDD) of World Health Organization(WHO) has been used for evaluating the amount of medicine use. However, DDD of some drugs must be determined for drugs in Korea which are not listed by WHO. Our formulary follows ourself classification and DDD of some drugs must be determined since they exist only in Korea. This study was aimed to determine DDD value using RAND Appropriateness Methods and evaluate the amount of antibiotics use using DDD value. Methods : J01 antibiotics of WHO anatomical therapeutic chemical(ATC) classification were extracted from drug formulary. Antibiotics list without DDD was identified to determine their DDD with comprehensive review of references and recommendation of experts. defined. Review of reference was executed. of Expert panels were comprised of clinical pharmacist and clinical doctors. Modified Delphi Method was applied by survey and consensus meeting. Amount of antibiotic use was calculated by DDD/1000 inhabitants/day in the national level using health insurance claim data. Results : The result of 1 round, DDD values of 28 ingredients were determined from the first round of consensus meeting. With 2nd round meeting, 3 ingredients were deleted and DDD of 17 ingredients were decided. Analysis of antibiotic use in health insurance claim data showed 22.97 DDD/1000 inhabitants/day in 2003 year. Conclusion : This study can contribute to the establishment of DDD assignment and thus quantifying drug uses.