Biodiesel offers cleaner combustion over conventional diesel fuel, including reduced particulate matter, carbon monoxide and unburned hydrocarbon (HC) emissions. However, several studies point to increase in NOx emissions (about 13%) for biodiesel fuel compared with conventional diesel fuel. In this paper, the experimental investigation on the effect of antioxidant additive (L-ascorbic acid) on NOx emissions in a methyl ester of cottonseed oil-(MECSO) fuelled direct injection diesel engine has been reported. The antioxidant additive is mixed in various proportions (100-400 mg) with MECSO and was tested in computerised four-stroke water-cooled single-cylinder diesel engine of 3.5kW rated power. Results show that the antioxidant additive is effective in controlling the NOx and HC emissions of MECSO-fuelled diesel engines.