Inclusion criteria for ASD children.  Children who meet Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) and ADOS-2 diagnostic criteria. Age 3-18 years.  Inclusion criteria for donor.  Children who match ASD children in age and gender, preferably from relatives. No antibiotic treatment in the last 3 months. Children who are healthy in growth and development, with normal results of child developmental and neuropsychological tests, including ASD-related screening, Attention-deficit hyperactivity disorder (ADHD)-related screening and motor screening. Children who have normal serological test results, including pre-transfusion testing, liver and kidney function, hepatitis B detection, quantification of hepatitis C, quantification of A virus immunoglobulin M (HAV-IgM), Epstein-Barr virus DNA (EBV-DNA) and Human Immunodeficiency Virus (HIV), detection of microvirus B19, TORCH, tuberculosis infection T-cell test (T-SPOT), total total immunoglobulin E (IgE), food and inhalant allergen testing, and lymphocyte subpopulation analysis. Children who have normal stool test results, including fecal routine and fecal occult blood, bacteria (Clostridium difficile A/B toxin, Escherichia coli O157, Shigella spp., Salmonella spp., Campylobacter spp., Staphylococcus aureus, Yersinia spp., Vibrio parahaemolyticus, Cholera isolates), fungi (Pseudomonas albicans, etc.), viruses (rotavirus, etc.), parasites (Giardia lamblia, Cryptosporidium, Cyclospora). Children who have normal results for chest X-ray, abdominal ultrasound, C13 breath test, cranial MRI, etc.  Exclusion criteria for ASD children.  ASD children with severe gastrointestinal symptoms or organic disease requiring immediate surgery or treatment. ASD children who have received antibiotics within 3 months, or are receiving immunosuppressive agents and biologics. ASD children with underlying diseases, such as severe anemia, malnutrition, autoimmune diseases (autoimmune thyroiditis, type I diabetes, etc.), allergic diseases (asthma, severe eczema, etc.), central nervous system diseases, metabolic syndrome, etc. ASD children with other organic dysfunctions, such as cerebral palsy, congenital genetic diseases, etc.; history of other psychiatric-behavioral disorders, genetic-metabolic diseases and other major physical diseases; other physical diseases, such as hearing impairment, vocal disorders, blindness, etc.  Exclusion criteria for donor.  Children who have functional gastrointestinal symptoms suggested by the Rome IV diagnostic questionnaire for functional gastrointestinal disorders. Children who have gastrointestinal disorders, including gastrointestinal symptoms (e.g., nausea, vomiting, abdominal pain, bloating, diarrhea, constipation, etc.), chronic gastrointestinal disorders (chronic diarrhea, chronic abdominal pain, etc.), gastroesophageal reflux disease, peptic ulcer, a history of gastrointestinal surgery (intestinal obstruction, megacolon, pyloric stenosis, etc.) Children who suffer from other diseases, including familial autoimmune diseases such as type I diabetes, inflammatory bowel disease, rheumatoid arthritis, chronic lymphocytic thyroiditis (Hashimoto's disease), toxic diffuse goiter (Graves' disease), etc.; Children who have received drugs that has impact on the intestinal microbiota (such as proton pump inhibitors, pro-gastrointestinal drugs, steroids, aspirin, etc.) within six months. Children who have received antibiotics within three months; Children who are receiving immunosuppressants and biological agents. Children who are undergoing chemotherapy for various tumors; Children who have liver and kidney diseases, central nervous system diseases, acute and chronic infectious diseases (tuberculosis, measles, syphilis, HIV, etc.), severely anemic, malnourished, metabolic syndrome (obesity, diabetes, etc.), Children who live in a place with prevalence of bacterial, viral, parasitic, etc.