Background and Objectives: There is a higher frequency of obesity in children with autism spectrum disorders (ASD). Their environment, such as their parents' attitude towards health matters may affect this issue, however, this link has not yet been well established. Thus, the aim of study is to examine how parental psychological factors, such as self-efficacy and outcome expectancy, affect the children's overweight/obesity status (ob). Methods: A cross sectional study involving sixteen schools for special needs education in Ibaraki prefecture and two schools in other areas in Japan was conducted. At the beginning of September 2022, a questionnaire on the parents' self-efficacy, outcome expectancy as well as a brief self-administered diet history questionnaire was distributed to parents of all first grade to sixth grade elementary school children. Fully completed questionnaires of children whose parents reported them as having ASD or autism, were then analysed. The exposure variable was ob or non-overweight/obesity (non-ob), assessed using self-reported height and weight measurements. The parents' self-efficacy and outcome expectancy were assessed using a five-ordinal scale. A logistic regression analysis was employed to estimate the odds ratio with 95% confidence interval for the analysis of effects of parental factors on ob in children with ASD. Results: There were a total of 147 participants (Autism; n=52, Asperger; n=0, ASD; n=95). It was observed that the parents of ob children have a lower outcome expectancy regarding the healthy diet and having breakfast than the parents of non-ob children, although no significant associations were observed in personal factors, such as school area and sex. Fifth grade children were more likely to be classed as ob than those in younger grades. Conclusions: Support, such as a school-based nutrition education program, particularly before fifth grade, may be required to improve parental attitude which influences the eating behaviour of children with ASD.