In this study, energy and exergy analysis of a direct injection compression ignition engine converted into dual fuel systems using different composition of mixed fuel formed from natural gas and diesel were performed. Effects of parameters, such as composition of mixed fuel, engine load, and speed, on various energetic and exergetic performance parameters of the engine were evaluated and compared with each other. The irreversibilities of mixed fuel and diesel were comparable at high engine speed but at low engine speed the irreversibility of mixed fuel was higher than that of diesel fuel.