This paper instroduces Feuerstein\'s theory of cognitive modifiability and his Instrumental Enrichment program Feuerstein\'s assumption of dynamic assessment has much in common with Vygotsky\'s concept of zone of proximal development. Ans experiment was also conducted in this study to ascertain the effectiveness of Feuerstein\'s IE program. The subjects were 12 intellectually low functionaing middle schhol boys. They ranged in age grom 12 tp 13 years at the beginning of the experiment. Their average IQ was 69 with range of 63~80 . Their socio - economis status was very low. Three instruments ( Organization of Dots. Orientation in Space I , and comparison) were used in this experiments with 40 minutes lesson a day, 3 days weekly for about 7 months. With the constraints of school administraion and policy, the lesson couldn\'t be given at the regualr school hours, So the lesson was conducted during the 40 minutes \" free- learning time \" early in the morning. All the lesson was given by a trained instructor. Therefore the lesson time and instructror\'s mediation were not sufficient because time and instructor\'s mediation had to be distributed among 12 boys. In addition to the observation of students\' progresses by both instructor and hoomroon teacher, changes in students\' cognitive functioning were monitored by pre-and post-assessment of Raven\'s Matrices. The result was very positive , thus supporting feuerstein\'s theory of cognitive modifiaability and evidencing the effectiveness of his IE program .