In a conventional diesel engine, fuel is injected into the combustion chamber at extremely high pressure shortly before the TDC (top dead center). This fuel is atomized and heated by contact with the hot charge in the cylinder. As a result, it rapidly undergoes pre-flame reactions and self-ignites. However, in Compressed Natural Gas (CNG)-diesel engine (dual fuel system), CNG will enter the combustion chamber along with the air intake. Since the diesel fuel provides only an ignition source rather than the main source of combustion energy, the amount of diesel fuel required is incomparable to CNG. CFD-ACE software is used to simulate the effect of different compression ratios on the engine combustion for CNG-diesel engine. The piston head is machined to reduce the original compression ratio. This paper investigated: 1) the quality of mixing in the combustion chamber for different compression ratios, 2) the quantity of the heat energy that dissipated to the walls of the combustion chamber using CFD model and 3) the affect of the compression ratios on the engine power and torque experimentally. It was found that the increase of compression ratio promotes homogeneously mixing of the CNG and air, also it was found that the break power and torque obtained from CNG-diesel engine is comparable with the conventional diesel engine. Governing equations, such as the kinetic energy, kinetics, mass transport and momentum equations, for a particle are used to simulate and design the upper part of the piston.