This paper study the NOx pollutants emitted from a single cylinder, 4-stroke spark ignition engine type Prodit; fueled with supplementary Natural gas to LPG. The study focused on the impact of equivalence ratio, spark timing, engine speed, and the added NG volumetric ratio on NOx emissions. The results showed that NOx levels depend mainly on the equivalence ratio, as the higher value of NOx concentrations was on the lean side near the stoichiometric equivalence ratio, and it was reduced when getting far from this ratio. It was observed that retarding spark timing reduces the NOx concentrations by a significant percentage. The NOx levels became higher at medium speeds and reduced in higher and low speeds. NOx levels increased with increasing NG volumetric ratio in the mixture.