Dynamic performance tests were carried out on a medium speed diesel engine, using light diesel fuel and heavy fuel oil. Cylinder pressure was measured to evaluate the combustion characteristics. The engine's dynamic performance corresponding to the parameters of two different injection nozzle were compared. The influence of fuel temperature on the dynamic performance of heavy fuel oil was studied. Results show that :if the nozzle is kept unchanged, using of heavy fuel oil causes higher pressure increasing rate and increased peak pressure under low and medium load conditions; on the contrary, under high load condition, peak pressure decreases; compared with light diesel fuel, the peak pressure gets higher for heavy fuel oil after increasing the nozzle hole number and enlarging the hole diameter; fuel temperature also has great impact on combustion performance of heavy fuel oil, and there is an optimal fuel temperature; with the increasing of fuel temperature, the combustion pressure gets higher, however, when the fuel temperature reach a certain limit, the dynamic performance of the engine turns to be worse.