Combustion studies on the use of vegetable oils as diesel fuel substitute were carried out. The use of neat vegetable oil in diesel engines presents some problems due to their chemical structure. High viscosity appears to be a fundamental property related to many problems associated with direct use of plant fuels in diesel engines. High viscosity has the effect of increasing droplet size on fuel injection. This results in poor combustion. The purpose of this research work was to evaluate the effect of elevating fuel inlet temperature on viscosity and the performance of rapeseed oil in a modern, unmodified diesel engine under part-load test conditions. The overall test results showed that fuel heating improved the combustion characteristics of rapeseed oil fuel. The brake specific fuel consumption (b.s.f.c.) was reduced and brake thermal efficiency was significantly increased compared to the baseline test on diesel fuel. For the three quarter-load operation, the high combustion temperature becomes the dominant factor on the performance of plant oil fuels, making both heated and unheated fuel acquire the same system temperature before fuel injection. The combustion chamber was free of abnormal carbon deposit though the lubricating oil test showed reduction in viscosity.