According to the increase of energy demand and environmental crisis, natural gas has been studied and considered as one of the clean alternative fuels. However, knocking phenomena is unavoidable when large diesel engines operate with natural gas. This research aimed to investigate a large diesel engine fuelled with diesel – natural gas dual fuel system. Firstly, two natural gas vaporizers were tested to find the appropriate one. Then, a comparative experimental between diesel operation and dual-fuel operation was conducted. Output performance was considered and found that vaporizer with 3.8-metre internal length heat exchanger was more advantageous. Thus, this vaporizer was installed in the tested engine. The experiment was done over the speed range between 1,100 to 2,000 rpm. It was found that dual-fuel engine produced less torque and power than those of diesel engine around 2.10%, which was still acceptable in actual application. The result found that the amount of injected natural gas must be controlled as a function of engine speed to avoid engine knocking. Averagely, the natural gas was injected into the system around 71.32%. This leaded to the reduction of engine volumetric and thermal efficiencies around 2.73% and 3.5% respectively.