For the purpose of analyzing the characteristics of the development of number concept in the educable mentally retarded, the task presented by Piaget was applied to the mentally retarded in Korea where the culture .and customs are different from those of his. The subjects of this study were total 240 . children; 120 the educable mentally retarded of Daegu Nam-e-yang Special School (lQ 54-70, MA 4:0-9:5, CA 7: 11-13:4), 20 children of Hyo Seong Kindergarten, 80 first-graders of Daegu Nam-e-Do Elementary School and 20 children of Bog Jaug Catholic Church Kindergarten. The main conclusions from the examination of this task are as follows: 1. The development of number concept begins at about same age for the retarded and the normal: 4 or 5 years of age. However it takes 2 years longer for the retarded (at the age of 9) than for the normal Cat the age of 7) for a full development of the number concept. There is no significent difference between the sexes in the development of number concept. 2. Three distinct developmental stages. Preformation, partial formation, full-formation are observed in each task but there is only partial correlations among the development of each task. a. In the task on conservation of the number concept (CQC), reversibility and complementarity is not fully formed untill the age of 10 for the mentally retarded and 8 for the normal. This task takes longer time than others for its full formation. b. The ability of matching two sets by one to one correspondence develops at about the same rate for that of counting and conserving numbers. The operational concept emerges at the age of 7 in the normal and at 9 in the retarded. c. In the task of transition of equivalent relation there is no difference between equivalent relation and transition in development. d. The development of sequence construction is followed by that of sequential correspondence. e. Cardinal number and ordinal numbers are not acquired at the same time. 3. The characteristic of the response pattern in the development of number concept is that the task of SOC, OOC, CQC have a sequential developmental pattern while others don’t have a strick one.