Biodiesel offers cleaner combustion over conventional diesel fuel, including reduced particulate matter, carbon monoxide, and unburned hydrocarbon emissions. However, several studies point to an increase in NOx emissions for biodiesel fuel compared with conventional diesel fuel. In this article, the experimental investigation of the effect of antioxidant additive (ethylenediamine) on NOx and HC emissions in a methyl ester of neem oil fueled direct-injection diesel engine has been reported. The antioxidant additive is mixed in various proportions with methyl ester of neem oil and was tested in a computerized four-stroke water-cooled single-cylinder diesel engine of 3.5 kW rated power. Results show that the antioxidant additive is effective in controlling the NOx and HC emissions of methyl ester of neem oil fueled diesel engines.