Background: Autism is one of the pervasive developmental disorders throughout the world that is causing serious familial, social and economic burden to the different nations. The prevalence of autism is probably about 20 per 10000 children. Autism is four times common in boys as in girls. Objective: To find out the psychiatric co-morbidity in Autism spectrum disorder. Methods: This was a descriptive and cross sectional study among the patient attending in Department of Psychiatry, Combined Military Hospital, Dhaka, Proyash School (Special Child Special Right), Dhaka Cantonment, National Institute of Mental Health (NIMH), Dhaka and Institute of Pediatric Neurodisorder & Autism (IPNA) in Bangabandhu Sheikh Mujib Medical University (BSMMU), Dhaka, Bangladesh. The study was carried out 6 months from April, 2016 to September, 2016. Sample size n=100 patients. Informed written consent was taken from patent’s parent/caregiver. During data collection, a semi-structured questionnaire designed by the researcher containing socio-demographic and other variables was used. Then Developments and Well-Being Assessment (DAWBA) was applied on the ASD children. The DAWBA has three versions-parent versions, self-version and teacher version. In this study parent version was used. Data was analyses by software program me statistical package for Social Sciences (SPSS) version 16 for windows using appropriate statistical test- chi square test. Results: Among the respondents (n=100) 79% Autism spectrum disorder (ASD) children had comorbid psychiatric illness and 21% children had no comorbid psychiatric illness. Most frequent 72.15% was ADHD followed by Specific phobia 11.39%, Social phobia 6.33% and Major Depressive disorder 1.27%. Two comorbid psychiatric illnesses were present in 8.86% ASD children. Chi-square test was done to measure the level of significance. But there were no significant association (significant when p value ≤0.05) found between socio-demographic factors and comorbid psychiatric disorder. Conclusion: Although there were many limitations, the present study provides important information. High number of co-morbidities found in this study. It could be necessary to increase the sample size and to collect the largest number of clinical information, for studying etiopathogenesis and risk factors for the occurrence of co-morbidity. Knowing the burden and extent of disease could be help design screening tools that are applicable, culturally acceptable and cost-effective for early diagnosis and intervention. Furthermore, raising ASD awareness among parents, preschool/elementary school teachers are invaluable in helping autistic children cope with different challenge and improve their quality of life.