Objective   To investigate the relationship between gut microbiota and severity of symptoms in children with autism spectrum disorder(ASD). Methods   Between April and July 2017, 45 children with ASD were enrolled in the Research Center for Psycho-Behavioral Development of Children and Adolescents at Sun Yat-sen University in Guangzhou, China. The childhood autism rating scale(CARS) and the social communication questionnaire(SCQ) were used to assess the severity of ASD symptoms. Stool specimens were collected from participants. The fecal microbial DNA was extracted from feces. We characterized the composition of the fecal microbiota using bacterial 16S rRNA gene sequencing. Results   There were no significant diversity and structure differences in gut

microbiota between mild-moderate ASD and severe ASD groups. However, the severe ASD group had a smaller number of operational taxonomic units, the rank abundance curve was steeper and the horizontal span was smaller. In addition, the relative abundance of Clostridium clostridiiforme was positively correlated with the restricted and repetitive behavior subscale score(r=0.391, P=0.008). Conclusion   Gut microbiota was similar in different severity ASD groups. However, the gut microbiota in severe ASD group trended to be less diverse and more uneven.