The objective of this study was to compare experiential grammar in the narrative discourse of thirty typically developing Thai children and thirty Thai children with autism in elementary grades 1 to 3. Data was compiled by asking subjects to tell a narrative discourse from a wordless storybook “A Boy, a Dog, and a Frog” (Mayer 1967). Data was analyzed based on the Systemic Functional Linguistics approach and focused on experiential mode of meaning concerned with the system of Transitivity. The results revealed that children with autism used fewer clause complexes and clause simplexes than typically developing children. For the system of Transitivity, it was found that children with autism used less modified nominal groups as the Participant especially in grade 2 and 3 and they used a smaller number and less variety of process types; used a lower percentage of mental, behavioral and verbal processes; and used a lower percentage of serial verb constructions. Children with autism, especially those in grades 2 and 3, used a smaller variety of circumstance types; and used a lower percentage of clauses with complex circumstances.