Abstract— The scarcity of world’s petroleum reserves and the sources are at the extreme of getting abolished. Due to the increase in the price of petroleum, environmental concern and availability of fuels are greatly affecting the trends of fuels for transportation vehicles. To fulfill the rising energy demand renewable fuel like biodiesel is in forefront of other technologies. Biodiesel oil one of the option as alternative transport fuel. In the present study, experimental investigations were carried out on a small size, variable compression ratio diesel engine with cotton seed -diesel blends (10–25% by volume) as fuel to determine the optimum blending ratio for the engine performance. Cotton seed oil is non edible vegetable oil, high viscosity, low volatility and widely available in India. It has been observed that B-20 blend (20% cotton seed +80% diesel fuel) gives highest brake thermal efficiency & lowest brake specific fuel consumption as compared to all other blending combinations at different compression ratios. Optimized B20 blend gives very close thermal efficiency, 2.46% lesser to diesel fuel at compression ratio 17 at 75% of maximum load i.e. 12 kg. Among all diesel blend maximum break power was found 3.295 kW by B20 blend at compression ratio 17.