"Ginkgo biloba" has been reported to affect the neurotransmitter system and to have antioxidant properties that could impact the pathogenesis of Autism Spectrum Disorder. Based on these studies, we decided to assess the effectiveness of "Ginkgo biloba" extract (Ginko T.D., Tolidaru, Iran) as an adjunctive agent to risperidone in the treatment of autism. Forty-seven outpatients with a DSM-IV-TR diagnosis of autism ages between 4 and 12 years were assigned to this double blinded clinical trial and were randomly divided into two groups. One group received risperidone plus Ginko T.D and the other received risperidone plus placebo. The dose of risperidone was 1-3 mg/day and the dose of Ginko T.D. was 80 mg/day for patients under 30 kg and 120 mg/day for patients above 30 kg. Patients were assessed using Aberrant Behavior Checklist-Community (ABC-C) rating scale and the side effect check list every 2 weeks until the endpoint. None of the 5 subscales of ABC-C rating scale showed significant differences between the two groups. Incidents of side effects were not significantly different between the two groups. Adding "Ginkgo biloba" to risperidone did not affect the treatment outcome of ADs. Nevertheless, further observations are needed to confirm this result.