Objectives: The Diagnosis Related Group (DRG) payment system, which has been mplemented in Korea since 1997, is based on voluntary participation. Hence, the positive impact of this system depends on the participation of physicians. This study examined the factors determining participation of Korean obstetrics & gynecology (OBGYN) clinics in the DRG-based payment system. Methods: The demographic information, practice-related variables of OBGYN clinics and participation information in the DRG-based payment system were acquired from the nationwide data from 2002 to 2007 produced by the National Health Insurance Corporation and the Health Insurance Review & Assessment Service. The subjects were 336 OBGYN clinics consisting of 43 DRG clinics that had maintained their participation in 2003-2007 and 293 no-DRG (fee-for-service) clinics that had never been a DRG clinic during the same period. Logistic regression analysis was carried out to determine the factors associated with the participation of OBGYN clinics in the DRG-based payment system. Results: The factors affecting participation of OBGYN clinics in the DRG-based payment system were as follows (p<0.05): (1) a larger number of caesarian section (c/sec) claims, (2) higher cost of a c/sec, (3) less variation in the price of a c/sec, (4) fewer days of admission for a c/sec, and (5) younger pregnant women undergoing a c/sec. Conclusions: These results suggest that OBGYN clinics with an economic practice pattern under a fee-for-service system are more likely to participate in the DRG-based payment system. Therefore, to ensure adequate participation of physicians, a payment system with a stronger financial incentive might be more suitable in Korea.