Objective : To gauge the gross and fine motor development of early treated phenylketonuria (ETPKU) in children in the age range of 1-4 years. Methods : A cross-sectional analytic study was conducted in PKU clinics (reference clinics for PKU follow-up), Tehran, Iran. Seventy children with ETPKU were selected as the case group for the study. ETPKU children were those with early and continuous treatment with a phenylalanine-restricted diet (the mean of blood phenylalanine level during the recent 6 months was 2-6 mg/dL or 120-360 μmol/L). Also, 100 healthy and normal children matched with the ETPKU group for age were randomly selected from 4 kindergartens in four parts of Tehran as a control group. The measurements consisted of a demographic questionnaire, Peabody Developmental Motor Scale-2 (PDMS-2), and pediatrician assessment. Motor quotients were determined by PDMS-2 and then compared in both groups by two independent samples t-test. Results : The mean ages in case and control group were 28.5 (± 11.6) and 29.7 (± 11.3) months, respectively. Comparison of the mean fine, gross, and total developmental motor quotients (DMQs) showed statistically significant differences between the two groups (p < 0.05). The fine and total DMQs of ETPKU children were also correlated with age. In addition, there was a negative correlation between the phenylalanine level and fine (p < 0.001) and total (p = 0.001) DMQs. Conclusion : It seems that ETPKU Iranian children, regardless of following a phenylalanine-restricted diet or not, have lower motor development. It is recommended to plan programs for early detection and intervention of developmental delays in these children.