Background: Autism is a developmental and behavioral pattern, the triad of impairments, 1. social interaction, 2. social communication, 3. imagination. Their memories are seemingly in picture or photo records. Difficulties in the treatment, management, and handling of autistic children are the main problems. Hyperbaric oxygen therapy (HBOT) is a modern treatment in Thailand for nitrogen imbalance (Decompression sickness syndrome or Caisson disease). HBOT can increase plasma oxygen to the tissues including the brain. Objective: To determine whether Hyperbaric Oxygen Therapy is safe to use in children with autism, and has a statistically significant effect on autistic symptoms. This is the first study in Thailand. Material and Method: Thai Autistic children (n = 7) received HBOT (1.3 atm., 10 sessions) treatment. Assessment was done before and after treatment in five domains: Social development, Fine motor and Eye – hand coordination, Language development, Gross motor development, Self – help skills. Results: Improvement was shown in five domains with a significant level. Seventy-five percent of children shown improvement while 25% did not seem to respond to the treatment. Conclusion: HBOT is a new treatment for Thai autistic children. Many scientific studies recently have shown that HBOT could be an effective treatment for autistic children. It could improve the major autistic symptoms.