Causes of autism are still unknown. Some studies have shown that autism might be associated with metabolic abnormalities in the folate/homocysteine pathway, which is involved in DNA methylation, thus altering gene expression. The association between the methylenetetrahydrofolate reductase (MTHFR) gene C677T polymorphisms and the risk of autism is still controversial and ambiguous. The purpose of this study was to examine the effect of the MTHFR C677T polymorphism on the autism risk in the Chinese Han population. A population-based case-control study was conducted in 186 children with autism and 186 controls. The MTHFR C677T polymorphisms were determined by using a polymerase chain reaction-restriction fragment length polymorphism assay. The frequency of genotype MTHFR 677TT in children with autism (16.1%) was significantly higher (odds ratio [OR]=2.04; 95% confidence interval [CI]=1.07, 3.89; p=0.03] than those in controls (8.6%). When stratifying by select-item scores on the Autism Diagnostic Interview-Revised, it was found that children with current overactivity had a significantly higher frequency of the MTHFR 677TT genotype (OR=2.77, 95% CI=1.17, 6.60; p=0.02) than those without. This study suggested that MTHFR C677T is a risk factor of autism in Chinese Han children.