This paper analyses injury mechanism involved in a frontal collision crash and the effect of seatbelt use to reduce injury severity. All evidences on vehicle, driver, and road were collected during accident investigation. This case is a fatal accident involving single car with single occupant hitting to roadside barrier. Two step analysis was conducted. Firstly, crash occurrence was reconstructed to estimate the parameters of impact. Secondly, models were developed to study the injury mechanism involved in the crash. Models with and without seatbelt use were developed to describe what happen to body of driver when the seatbelt is used and not used during the crash. Analysis results show that seatbelt use can reduce injury severity.