This paper investigates the performance and emission characteristics of a diesel engine with mustard oil and its diesel blends. The mustard oil-diesel blends M5 (5% Mustard oil and 95% diesel), M10 (10% Mustard oil and 90% diesel), M15 (15% Mustard oil and 85% diesel), and M20 (20% Mustard oil and 80% diesel) was prepared to test in diesel engines. The present experimental results were obtained on the performance and the emissions of CO, HC and NOx in diesle engine. Comparison of mustard blends (M5, M10, M15 and M20) with engine diesel was done. The results showed that the brake thermal efficiency was decreased as the blend increased, and the brake specific fuel consumption was slightly higher than the diesel fuel. The CO and HC emissions are higher than diesel. However, NOx emissions of the blends were found to be decreased significantly compared to diesel as blend ratio increased. Smoke emission was found to be increased slightly when compared to diesel.