Due to the vast resources of Natural Gas (NG), it has emerged as an alternative fuel for SI internal combustion engines in recent years. The need to have better fuel economy and less emission especially that of greenhouse gases has resulted in development of NG fueled engines. Direct Injection of Natural Gas into the cylinder of SI Internal Combustion Engines has shown great potential for improvement of performance and reduction of engine emissions especially CO2 and PM. Direct Injection of NG into the cylinder of SI Engines is rather new, thus the flow field phenomena and suitable configuration of injector and combustion chamber geometry has not been investigated completely.In this study a numerical model has been developed in AVL FIRE software to perform investigation of Direct Natural Gas Injection into the cylinder of Spark Ignition Internal Combustion Engines. In this regard, two main parts have been taken into consideration, aiming to convert an MPFI gasoline engine to direct injection NG engine. In the first part of study multi-dimensional numerical simulation of transient injection process, mixing and flow field have been performed via three different validation cases in order to assure the numerical model validity of results. Adaption of such a modeling was found to be a challenging task because of required computational effort and numerical instabilities. In all cases, present results were found to have excellent agreement with experimental and numerical results from literature.In the second part, using the moving mesh capability, the validated model has been applied to methane Injection into the cylinder of a Direct Injection engine. Five different piston head shapes have been taken into consideration in investigations. An inwardly opening single-hole injector has been adapted to all cases. The injector location has been set to be centrally mounted. The effects of injection parameters, combustion chamber geometry and engine RPM have been studied on mixing of air-fuel inside cylinder via quantitative and qualitative representation of results. Based on the results, suitable geometrical configuration for a NG DI Engine has been discussed.