In this work, an experimental investigation was conducted in order to study the effect of using methanol- diesel blends on the steady state performance of diesel engines. test rig was comprising a four stroke 5.67 liters, water-cooled diesel engine and a Heenan hydraulic dynamometer. Instrumentation included devices for measuring engine speed, load, exhaust gas temperature, NOx, in addition to fuel and air consumption rates. Tests were carried out on the engine when running with diesel fuel first, and then with methanol- diesel blends at different methanol concentrations, namely 2.5, 5 and 15 % by volume. The results showed that methanol concentration has no effect on exhaust temperature, while NOx formation was affected by methanol concentration in the fuel blend. This effect was limited to 2.5 % methanol concent-ration by volume.