Analysis of different methods of diesel engines conversion for operation on natural gas was carried out showing feasibility of these methods for engines of various applications. The KAMAZ V8 120/120 mm diesel engine was converted to operate on natural gas by spark ignition cycle and Cummins Kama 6L 104/127 mm diesel engine – by gas diesel (dual fuel) cycle. Gas feed and electronic engine control systems were developed for both the methods of engine conversion. For gas diesel version, modular gas feed and engine control systems were developed which could be mounted both on high-speed and medium-speed gas diesel engines. These systems were perfected during engine tests. CR fuel system for the perspective medium-speed D200 6L 200/280 mm gas diesel engine was developed jointly with the industrial partner. A computer model for simulation of engine operation was developed and calibrated on the basis of engine tests and used for the development of gas feed and engine control systems, experimental results analysis and forecasting of the parameters of the medium-speed D200 gas diesel engine. Engine tests of the gas and gas diesel engines demonstrated considerable decrease of CO2 and NOx emissions, practically no soot and lower fuel consumption compared to the base diesel engine.