Abstract: The combustion process of diesel has a direct effect on engine power, economy and emissions. Moreover, the atomization quality of fuel injection is the most important factor affecting combustion process. Therefore, spray characteristics of bio-diesel，diesel and the mixture of bio-diesel and diesel were studied in this paper. Spray field of bio-diesel and mixed fuel in a model cylinder were measured by PDPA (Phase Doppler Particle Analyzer). Test data such as distribution of velocity, diameter of oil drop and concentration in the spray field were obtained. The results show that the diameter of nozzle orifice gives a great impact on atomization beyond the core area of the spray. The smaller the orifice diameter, the better the spray atomization. The injection pressure have little influences on the spray field of bio-diesel. Compared with fossil diesel fuel, bio-diesel performed worse spray characteristics. SMD (Sauter mean diameter) of bio-diesel is bigger than that of fossil diesel fuel under the same condition. The higher the pump speed is, the smaller the diameter and fluctuation velocity of droplet are.