Abstract: The fuel spray characteristics are key factors that affect engine combustion and emission. The spray characteristics of diesel and biodiesel were contrasted and analyzed. The factors such as density, viscosity, molecular structure, blend ratio, speed of injection pump and fuel temperature which have influence on spray characteristics were investigated. The results showed that the spray characteristics could be improved by decreasing the density, reducing viscosity and interrupting the double bonds. The penetration length of diesel and biodiesel reached maximum at 0-0.9 ms and 0.5-0.7 ms, respectively, when the speed of injection pump was 1 100 r/min. The SMD （Sauter mean diameter） of biodiesel reached maximum around 5°CA when the temperature was 380 K and the load was 100%.