Alternative fuels from different sources are very much in need for transportation sector mainly due to hazardous emissions from engines using convention fuel and their harmful effects on the environment. Biofuel from nonedible oil can be used as alternative fuel in CI engines. Sargassum a brown sea weed available in plenty in ocean is good source of alternative fuel. Biofuel prepared from this can be used as fuel in CI engines. In this study bio-oil prepared from sargassum is used in four stroke CI engine. Results shows improvement in performances when blended fuel are used, Brake thermal efficiency was found to be 19.81% when 10% blended fuel is used which is higher in comparison to diesel which is 17.92%. Brake specific fuel consumption was little less in comparison to diesel.