Purpose: Motor function is an important aspect of child development. Motor development is usually assessed in terms of age of achievement of motor milestones. Few previous studies have focused on motor development using WHO indexes particularly in Iran.Methods: In this cross-sectional study, age of achievements of six motor milestones was assessed in 800 children attending health centers of Dezful. Six milestones (sitting without support, hands-and-knees crawling, standing with assistance, walking with assistance, standing without support, walking without support) were assessed by mother self-reports.Results: The mean age of children was 26.3 14.6 months. Based on percentile 97 WHO, 12.4 % of children had delayed motor development. The logistic regression test showed a statistically significant association between gestational age and delayed motor development (p<0.001).Conclusion: Prevalence of delayed motor development is high. Further cohort studies with bigger sample sizes should be designed.