Autism spectrum disorder (ASD) is a neurodevelopmental disorder that can cause significant social, communication and behavioral deficits. Probiotics are regarded as active microorganisms. With sufficient amounts, probiotics can regulate intestinal flora, intestinal permeability, inflammation and antioxidant reactions in the body. These reactions may further promote health, regulate metabolic disease progression and prevent complications. Lactobacillus plantarum PS128 (PS128) was reported to be a psychobiotic in several animal and human studies which modulated the levels of neurotransmitters in different brain areas. This study is to evaluate whether the consumption of PS128 can improve the symptoms of patients with ASD. The current randomized, placebo-controlled trial was conducted to investigate the psychophysiological effects of PS128 in school children with ASD.