Working memory was specifically examined in 11 low-functioning autistic children by using the CAST computer game. The CAST game consists of two conditions which offer to examine different components of executive function. The subjects were divided into two subgroups (high- or low- scoring) according to their performance and heterogeneity within the autistic sample was found. The performances of the highscoring autistic subjects were not significantly different from the normally-developing controls children. However, the executive controls, such as planning and application of strategy were impaired in the high-scoring children with autism and NSDD. All the low-scoring subjects were with autism or NSDD, who were impaired in working memory capacity, but intact in spatial memory. An interaction between working memory and perseverative inhibition was also found. Results suggest that perseverative behaviour is a defining feature of autism, and working memory deficit is possibly accompanied with some neurological impairments rather than autistic specific.