Objective: This case report aims to discuss the diagnosis of mental retardation as insanity defence in a crime offender. Methods: We report a gentleman who committed murder and rape 9 years ago, and currently being treated at a mental institution. Results: Patient was certified to have mental retardation, and was pleaded on the defence of unsoundness of mind because he had defect of reason at the time of alleged offence. Conclusion: Mental retardation does fulfil the McNaughton’s rule. Unsoundness of mind becomes the insanity defence even for murder under section 84 of the Penal Code. ASEAN Journal of Psychiatry, Vol. 15 (1): January - June 2014: 97-100.