In Singapore, about 13% of all crashes are heavy vehicle involved during the period of 1998 – 2002. However, if crash severity is considered 9.41% of this particular type of crashes are either fatal or serious. On the other hand, during that period car involved crashes constitute 42.39% of all crashes. Among them only 5.16% are fatal or serious accidents. From the above discussion it is clear that though heavy vehicle crashes do not occur in a large number in Singapore their consequences is worse than other types of crashes particularly car involved crashes. It is clear that the situation of heavy vehicle involved accidents in Singapore requires attention. Clearly, for developing countermeasures in reducing the road crashes and the consequent injuries, there is a need for better understanding about the relationship between risk factors and the injury severity of heavy vehicle crashes in Singapore. The main objective of the study is to understand the relationships between risk factors and accident severity for heavy vehicle accidents and specially for large truck and bus-involved accidents. The study also suggests possible causes for the identified critical factors based on the engineering judgment.  In order to achieve the objective a suitable statistical model has been selected. A model is developed using accident data from 1998 to 2002 to establish a relationship among general characteristics, driver attributes, road condition, vehicle types as well as environmental condition with accident severity. The model is then calibrated and analyzed to find the critical factors. The factors which increase the severity of heavy vehicle involved crashes are type of Time of the day (night time), Area (factor, near by school), Type of location (bend), Heat and run, Road engineering (narrow road way) Surveillance camera, Nature of accident (bet. moving vehicle, bet.veh. & stationary object, bet. veh. & Pedestrian), Maneuvers of veh (turning R-way), Damage of vehicle (total wreck), Day of week (weekdays). On the other hand, Time ternd, Area (shopping complex, public house), Road engineering (sharp turn), type of collision (head to rear), Maneuvers of vehicle (turning L-way), Offending party (non offending party), Road surface (sandy) decrease the severity of crashes.