Propane gas could be used adding to the combustion air in diesel engines to improve engine performance and reduce emissions. In the experimental study, the propane effect has been investigated in terms of the engine performance. The experiments were conducted on a four-stroke, 3-cylinder direct injection diesel engine, at constant engine speed, different fuel mixture ratios and engine torques. In the diesel engine, propane was added to the combustion air in the amount of 250 g/h and 500 g/h. Engine torques are considered as unloaded, 9,81 Nm, 19,62 Nm, 29,43 Nm and 39,24 Nm (maximum load), respectively. It has been observed that as the propane addition increases with increasing load, the pressure inside the cylinder increases, there is a gradual decrease in the exhaust gas temperature. Increasing the addition of propane for the same torque caused raise in BSFC significantly. Therefore, the addition of propane affected fuel consumption negatively. Increasing the torque has also increased the efficiency. However, considering the same torque value, the increase in the propane ratio in the fuel mixture decreased the efficiency.