The present work aims to study the effect of adding an antioxidant additive (L-ascorbic acid) to orange peel oil biodiesel (OB20, 20% orange oil methyl ester + 80% diesel) in a diesel engine to decrease the exhaust emissions. Orange peel oil was extracted from the waste orange peels by solvent extraction method. Biodiesel was prepared by alkali transesterification process from the orange peel oil. Fuel samples are prepared with different percentage of antioxidant additive into biodiesel, 95% biodiesel + 5% L-ascorbic acid (OB20L5) and 90% biodiesel + 10% L-ascorbic acid (OB20L10). The experimental investigation reveals that the oxides of nitrogen emissions are significantly reduced for OB20L5 and OB20L10 when compared to OB20 and diesel fuel. Besides, the addition of an antioxidant additive to biodiesel in the engine slightly decreases the brake thermal efficiency (BTE) when compared to OB20 noticeably. Also, it was noted that the addition of antioxidant additive to OB20 slightly increases hydrocarbon, carbon monoxide and smoke emissions. © 2019, © 2019 Taylor & Francis Group, LLC.