Children visiting Paediatric Neurology clinic with a diagnosis of ASD and fulfilling inclusion , exclusion criteria will be recruited.  Informed consent will be obtained for all study participants. Participants will be randomised into two groups using randomisation software. Participants in one group will receive sodium valproate and participants in the other group will receive placebo. Sodium valproate syrup and placebo syrup will be dispensed in identical bottles.  Sodium valproate will be started at 10 mg/kg/day followed by increment of 10mg/kg every week until 40mg/kg/day dose is reached at the end of 4th week. Stable dose of 40mg/kg/day will be maintained for 6 months. Baseline and follow up assessment of complete blood count, liver function test, thyroid profile will be done as part of routine management for children with autism spectrum disorder and cost will be borne by patient.All children will continue to receive standard of care in the form of behavioural and speech therapy. Primary outcome – improvement in severity of autism will be assessed by CARS-2 and Aberrant Behavior checklist (ABC) scores at baseline and at 6 months after reaching stable dose of study drug.  Epileptiform discharges in EEG ,  Pediatric Quality of Life( PedsQL) and Children’s Sleep Habit Questionnaire (CSHQ) scores at baseline and at 6 months after reaching stable dose of study drug will be obtained to measure secondary outcome.  Telephonic interview will  be conducted at every weekly  after initiation of study dug and clinic visit at the end of 6 months.  Telephonic interview will be conducted by the principal investigator and will be enquired regarding any adverse event .  Patients will be asked regarding weight gain, decreased appetite , vomiting , jaundice, cold intolerance, decreased activity. If there is any adverse event at any time during study period, will be recorded and addressed.  If there are serious adverse events or adverse events of special interest related to study drug, the study drug will be discontinued.  At the end of study period and statistical analysis, the decision regarding continuation or discontinuation of anti-seizure medication will be decided and discussed with the parents.