Abstract: Efforts are being made throughout the World to reduce the consumption of liquid petroleum fuels wherever is possible. Biodiesel is recently gaining prominence as a substitute for petroleum based diesel mainly due to environmental considerations and depletion of vital resources like petroleum and coal. According to Indian scenario, the demand for petroleum diesel is increasing day by day hence there is a need to find out an appropriate solution. This study investigates influence of injection timing of 20% blend simarouba biodiesel on performance and emission characteristics. The effect of varying injection timing was evaluated in terms of thermal efficiency, specific fuel consumption, carbon monoxide, hydrocarbons and oxides of nitrogen. By retarding injection timing brake thermal efficiency can be improved of S20.