This study is to identify the effects of ability grouping on the changes in classroom instruction ( the level and the amount of instructional content, intructional speed, instructional methods), teachers\' instructional motivation and expectations of student achievement, students\' attitudes and classroom interactions and ultimately student\'s academic achievement. This kind of study is needed to examine the merits and demerits of academic ability grouping which has begun to be introduced to secondary schools in Pusan in 1996. The data were collected by using a questionnaire from 1,440 students in 15 academic high schools in Pusan, Seven schools of them had adopted the ability grouping instrucion in 1997 for some selected subjects including mathematics. The target subject of survey was mathematics, The effects of ability grouping was estimated in three ways. The first one is to compare the demendent cariables between the students who had partivipated in ability grouping instruction and those who had not. The second one is to compare the classroom instructions between the higher ability group and the lower ability group withing the schools which had adopted the ability grouping instruction. The way of identifying the effects of ability grouping is to check the level of congruence in estimating the effects of ability grouping. The results are summarized as follows: 1. Since the adoption of ability grouping there were almost no changes in the level and the amount of instrucional content, the instructional speed, and the level of student understanding of the content. Some exceptions are that the level and the amount of instrucional content went up in a slight degree only for the higher ability group while the lower ability group did not show any changes. 2. Since the adoption of ability grouping instruction there were not any changes in the variety of instrucional methods including the amount of time spent on the question and anwer between students and teacher. 3. After the ability grouping instrucion there sere almost no changes in the teacher\'s passion for instrucion and expectations on student achievement partivularly for the lower ability group . 4. The ability grouping instrucions does not appear to improve the classroom atmosphere for learning after the ability grouping was introdeced. The students\' disruptive bevaviors and indifference to teaching were still somewhat noticeable and students\' attitudes on classroom learning were somewhat negative . These attitudes and behaviors on classroom learning does not appear to have changed since the ability grouping had been introduced. 5. The introduction of ability grouping to classroom instruction does not seem to have afftected students\' affective attiftudes on mathematics learning and peer group interactions They include the interest and confidence in mathematics and the inferiority complex on mathematics achievement compared to peer students. 6. Finally, the instroduction of ability grouping does not appear to have changed academic achievement of students.