Introduction: Autism Spectrum Disorder (ASD) is a group of neuro-developmental disorders with a genetic/environmental origin. Recently, vitamin D deficiency is considered as a possible risk factor in ASD development. This study aims to compare the vitamin D serum level in children with and without autism spectrum disorder.
 Materials and Methods: In this case-control study, 13 children (3 to 12 years old) referred to the outpatient clinic of Ibn-e-Sina psychiatric hospital in Mashhad in 2014 who received the diagnosis of ASD  based on clinical interview according to DSM-IV by child and adolescent psychiatrist and score ≥30 in Childhood Autism Rating Scale were selected as the case group. Other axis I psychiatric disorders were ruled out. These children were divided into mild, moderate, and severe groups. 14 normal children without any medical or psychiatric disorders who were matched in age and sex with ASD children were selected as control group. Vitamin D serum levels were measured in the two groups through ELIZA technique. Data were analyzed by SPSS software, through descriptive statistics, Kolmogrov-Smirnov, Mann-Whitney and Pearson tests.
 Results: The mean vitamin D level in case group was 13.00 (distance between quartile: 9.6-19.5) and in control group was 12.00 (distance between quartile: 4.9-13.2). The difference between the two groups was not significant (P=0.350). Also, vitamin D serum levels had no significant relationship with the severity of ASD (P=0.534).
 Conclusion: Based on the results, the serum levels of vitamin D in children with and without ASD have not any significant difference and the level of this vitamin has not significant relation with the severity of ASD.