Biodiesel fuel is a liquid biofuel produced by chemical process form new and used phytogenic oils, animal fats. Biodiesel fuels can be utilization alone or mixing with the pure diesel at different proportion.  In the present work a diesel engine type (FIAT) , four cylinder, variable speed, direct injection was operated by sunflower oil methyl ester , biodiesel at different blend ratio . five different ratio of biodiesel blends 10%, 20%, 30%, 40%, and 50% by volume is used in this study and compared with using of pure diesel at variable loads and variable engine speed. The effect of biodiesel additive to pure diesel on the performance and emission characteristics. Adjust the engine speed at 1100 rpm by means of the engine tachometer and digital tachometer, and reduce the load gradually until the engine speed increased to 1900 rpm automatically by increments of 200 rpm. The BSFC for B20 It seems less than the other ratio of biodiesel blends, and the BTE of biodiesel blends is lower than the pure diesel but the B20 having high BTE in comparison with the other biodiesel- diesel mixtures. the UHC and CO emission for B20 is less than the biodiesel blends and pure diesel, but the NOX emission for B20 is lower than the other biodiesel blends and higher than pure diesel. The present work shows the B20 relatively is a better performance and combustion characteristic than that biodiesel blends ratio.