Purpose: The Functional Independence Measure for Children (WeeFIM) and the Pediatric Evaluation of Disabilities Inventory (PEDI) are the most commonly used measures of functional performance In children. Our study was performed to determine the reliability and concurrent validity of the instruments in mentally retarded children. Material and methods: The subjects consisted of 50 children with an age range of 9-87 months who were patients in an outpatient clinic. Demographic data, classification of medical and educational of children were recorded. After computing total scores and subscale scores of the two tests, internal consistency and Inter-item correlation were examined for reliability. Concurrent validity was assessed with Pearson correlation coefficients. Results: Internal consistency and inter-item correlation of PEDI was similar with WeeFIM. Pearson correlation coefficient between two tests was moderately high in communication/social function (0.766), high in self-care (0.898), very high In mobility (0.912) and total scores (0.930). Internal validity and the relation score between subgroups of the PEDI were found 0.93-0.98; WeeFIM 0.75-0.97. Conclusion: The high correlations indicate that the two tests are similar measures. Future assessments of these tests, standardization of Turkish population, the reliability and validity must be done in variable developmental disabled children.