Tests have been conducted on a special single cylinder-indirect injection-compression ignition-research engine (Ricardo- E6) fueled with gaseous fuel to investigate the effects of operating conditions such as concentration of gaseous fuel, quantity of diesel (pilot) fuel injected, injection timing of pilot fuel and exhaust gas recirculation (EGR) ratio on the NOx emissions. Diesel fuel was used as the pilot fuel and methane or propane were used as the main fuel which was inducted in the intake manifold to be mixed with the intake air. Through the experimental investigations, it is shown that, NOx emissions increase significantly by increasing the concentration of the gaseous fuel (total equivalence ratio), employing a large pilot fuel quantity, heating the inlet gaseous fuel-air charge and advancing the injection timing of pilot fuel. It was found that admission of diluents resulted in reductions in exhaust oxides of nitrogen (NOx). When carbon dioxide was additional to the inlet gaseous fuel-air charge, large reductions in NOx was observed. Finally, NOx emission was reduced with increasing the EGR ratio due to the corresponding reduction of oxygen concentration required for NOx formation.