This paper aims it finding the effect of increase of the ambient air temperature before entering the naturally aspirated diesel engine on specific fuel consumption. The experimental investigation was carried out on a diesel engine four strokes, water cooled and indirect injection. The experiments covered all tests. The tests included heating of the inlet air temperature by designing an electric heater then increasing the air temperature entering into the diesel engine. The results showed that brake specific fuel consumption increased with increasing inlet air temperature. Also the results showed that the brake specific fuel consumption decreased with increasing the brake mean effective pressure. Experimental data obtained in this work were compared with other references were found to be in good agreement with experimental results.