In Korea, breastfeeding practice has decreased since 1980's. According to the literature review, insufficient milk supply (IMS) is a major reason found by mothers for early termination of breastfeeding practice. Insufficient milk supply syndrome cannot be explained simply. IMS is a complex phenomenon that has never been adequately investigated in Korea. Using the modified IMS conceptual framework by Hill and Humenick(1989), a study was conducted at one well baby clinic located in metropolitan hospital. Approval was obtained from head of nuring service and employees in the well baby clinic. The study sample was to restricted mothers who initiated breastfeeding and still continuously breastfeeding (n=52) and mothers who initiated breastfeeding but terminated at the time of data collection (n=39). Factor analysis suggested that Potential Determinant factors, maternal psychologic factors were: sociocultural factors, breastfeeding behaviors, breastfeeding frequency, social support, maternal education and prenatal preparation, physical and sibling support, maternal confidence, and maternal physicals which accounted for 71.12% of the variance. Using discriminant analysis those potential determinant factors predicted 72.49% of the cases accurately. These findings of research suggest that the modified IMS model is valid. But additional variables which consider Korea sociocultural factors may need to be included in future studies to determine and develop an IMS model for Korea.