Using natural gas as an alternative fuel will be reduced exhaust emissions and dependence on petroleum. The practical solution for Vietnam's conditions is to convert diesel engines into natural gas engines. Understanding the effect of piston geometry on the performance of converted CNG engines helps domestic manufacturers come up to spark ignition natural gas engines with high performance and low emission. In this work, three different types of combustion chamber were made from milling piston head and varying thickness of cylinder head gasket. The experiment results showed that torque and power of ε = 11.5 were more significant than ε = 12.5. During experiment processes, the stable operation state of the converted engine with ε = 12.5 was challenging to control, and the noise levels were intense. The results of mass fraction burned were presented the effect of squish gap on performance was more substantial than that of the squish area. It can be concluded that for modifying the original diesel engines to run on CNG fuel, the maximum compression ratio should be set to 12.5 and must be redesigned accordingly to use the natural results.Implications: This manuscript shows the new technology to convert traditional engines into engines fueled by renewable energy. It contributes to reducing the fossil fuel crisis and environmental emissions.