Objectives : The present research was conducted to study the effect of sandplay therapy on social cognition development of educable mentally retarded children. Method: The present study was a pretest-posttest with control group. To measure the dependent variable social cognition, Kay Cognitive Diagnostic Test (KCDT) was used in both pretest and posttest sections. The sample was selected randomly in which included 16 people in experimental group (8 girls and 8 boys) and 16 in control group (8 girls and 8 boys). Then during 12 sessions sandplay therapy employed on experimental group and finally the posttest was administered to both experimental and control groups. The data were analyzed using univariate analysis of covariance. Results: The mean score of cognition development in experimental group was 1.07 and in control group it increased 0.06. There was a difference between the posttest mean scores of experimental and control groups in social cognition development (p<0.001). Conclusion: S andplay therapy can increase social cognition development of educable mentally retarded children.