The activities and grouping patterns ocurred in the Classrooms are clearly the most critical factors in distinguishing qualitative features of preschool program, However, in reality, no such specific information has been available for those concerned with qualitative preschool program, Thus the purpose of this study was to discover whether there is significant nelationship between the actvity and grouping pattern and quality of preschool center. The sample for this study consisted of 31 preschool centers. For each center, fifteen children were randomly selected. The total number of children in this study was 465. The istruments which were utilized to collect the data were the Classroom Check List, the Preschool Inventory Test, and the School Readiness Test. Changes in children's Preschool Inventory Test score during 5-month period were used as an indicator of relative developmental gains. The School Readiness Test were simply applied on time. The Classroom Check List was used to collect time-sampled observational dafa of staff and chidren. ANOVA was used to analyze group differences in pre-and post-test and School Readiness Test. Further the Stepwise Multiple Regression technique was applied to determine the relative effectiveness of independent variable contributing to developmental gain. Findings indicated that the activities and grouping patterns seemed to have effects on the quality of preschool centers. Especially, grouping patterns were critical determinants of qualitative features of preschool centers.