This paper deals with the experimental and numerical investigation of a 2.0 litre single cylinder Heavy Duty Diesel Engine fuelled by natural gas and diesel oil in Dual Fuel mode. Due to the gaseous nature of the main fuel and to the high compression ratio of the diesel engine, reduced emissions can be obtained. An experimental study has been carried out at three different load level (25%, 50% and 75% of full engine load). Basing on experimental data, the authors’ methodology is based on the use of one-dimensional and 3-dimensional models. The former is able to perform the whole engine with faster simulations while the latter can study deeply the even more complex phenomena (turbulence, combustion, etc.), due to the presence and to the interaction of the two fuels, which occur in the cylinder for the most interesting operating conditions. Thus the 3-D model in KIVA-3V code has been calibrated in order to fairly reproduce the pressure curves and consequently to reach the appointed value of Indicated Mean Effective Pressure which is the index of the load level. Then the results obtained from the CFD simulation in terms of fuel oxidation rate for the two distinct fuels have been used to build the combustion law in the one-dimensional model with the Ricardo WAVE software. Finally correlations have been found to allow the simulation of a wide range of operating conditions in the one-dimensional environment.