The aim of this study was to examine the effects of collaborative learning program on mentally retarded children's communication ability (understanding, expression, reading • writing). To achieve this purpose, the following hypotheses were built in this study on the basis of antecedent research. 1. The communication ability will be developed more for the particip- ants mentally retarded children than for the non-participant mentally retarded children in the collaborative learning program. 2. The understanding ability will be developed more for the participant mentally retarded children than for the non-participant mentally retarded children in the collaborative learning program. 3. The expression ability will be developed more for the participant mentally retarded children than for the non-participant mentally retarded children in the collaborative learning program. 4. The reading • writing ability will be developed more for the particip- ant mental retarded children than for the non-participant mentally retarded children in the collaborative lean1ing program. The subjects of this research consists of 12 mental retarded children in special early school located in seoul. 6 of subjects took part in the collaborative learning as experimental group, and the others did as control group. The subjects of experimental group attended in the 24 sessions for 9 weeks. The length of each session was for about 40 minutes. Each session of the instruction in experimental group was carried out by teacher-children group, then by children-children group, the last, and teacher ti1lked about that task with children. And the procedure of this study was Pre-test, training, and Post-test in sequences. The statistical methods to examine the effect of this study was t-test. The statistics of this research were processed by SPSS/PC. The findings of this study were as follows : First, The collaborative learning program has affected the development of mentally retarded children's communicational ability. Second, The collaborative learning program has affected the development of mentally retarded children's understanding ability. Third, The collabora- tive learning program has affected the development of mentally retarded children's expression ability. Finally, The collaborative learning program has affected the development of mentally retarded children's reading 〮 writing ability. But, the findings has not significant differance on the statistics.