Introduction: Balance disorders are major motor complications in children with autism spectrum disorder (ASD). Physical exercise can improve motor complications in patients with this disorder. The aim of this study was to investigate the effect of basic gymnastics exercises on the postural stability in children with autism spectrum disorder. Materials and Methods: 20 boys with autism spectrum disorder participated in a controlled pre-post clinical trial study. Participants were divided in two 10-participant groups of case and control based on their scores in Gilliam Autism Rating Scale-II. The participants in the case group were trained with basic gymnastic exercises for 12 weeks (three sessions per week), each session lasted 45 minutes. The control group did not participate in any exercise during this time. Postural stability was measured using a force plate. Data were analyzed using multivariate ANOVA test while significant level was set at 0.05. Results: In post-test, less center of pressure oscillation was observed in the case group (657 ± 143 mm) compared to control group (1057 ± 311 mm) (P = 0.013). In the case group, the center of pressure oscillation was significantly reduced after gymnastic exercise (P = 0.001); but the changes were not significant in control group (P > 0.05). Conclusion: It seems that gymnastic training improves the postural stability through the reduction in the length of postural sway.