OBJECTIVE: To assess motor development of preterm and term infants using the test of infant motor performance (TIMP) and to compare the reliability of the TIMP measuring by different raters. METHOD: 55 preterm and term infants were tested with the TIMP at ages ranging from 35 weeks postconceptional age to 14 weeks corrected age. A linear correlation analysis was used to assess the contributions of age, risk, and radiologic findings to the variance in TIMP scores. In the reliability study, the TIMP was administered to the infants by an occupational therapist, and infant performances were videotaped. The performance was then rescored by another tester to examine the interrater reliability. RESULTS: No significant correlations between infant variables and TIMP score changes were revealed. In the reliability study, statistically significant correlation between two raters was found (Cohen's kappa: 0.959, p<0.001). CONCLUSION: TIMP has a good interrater test reliability for use in clinical practice to assess infant motor performance. But the TIMP could not discriminate among infants with differing risks for motor developmental delay. Further evaluation and follow up survey is necessary to find correlation between the development process and medical risk factor among the participated subjects in this study.