It studies the optimal proportion of biodiesel blended with diesel by an engine test．It used the uniform design method to design experimental programs，established the quadratic polynomial stepwise regression model of testing data，and with economic characteristics，dynamic characteristics and emission characteristics as indicators for the evaluation，found out the optimal proportion，based on the evaluations individually optimized and comprehensively optimized．The results show that CO，smoke and power reduce gradually，but fuel consumption and NO increases with increasing proportion of bio-diese1．And the optimal proportion of bio·diesel is 25％ ．