In diesel engines, adding natural gas to the combustion air can be a solution to increase engine performance and reduce emissions from combustion. In the experimental study, the effect of natural gas addition to the combustion air on the engine performance has been investigated. The experiments were carried out on a four-stroke, 3-cylinder direct injection Lombardini LDW 1003 engine, at constant engine speed, different mixing ratios and different engine loads. In the diesel engine, natural gas (NG) was added to the combustion air in the amount of 250 g / h and 500 g /h. Engine loads 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 natural gas addition increases with increasing load, the pressure inside the cylinder increases, there is a gradual decrease in the exhaust gas temperature, there is a decrease in diesel fuel consumption due to the natural gas addition. Carbon dioxide (CO2), carbon monoxide (CO), unburned hydrocarbon (HC) and nitrogen oxide (NOx) emissions were investigated for all test cases. Especially the amount of NOx emission increases with increasing natural gas addition up to a certain load value.