Abstract: Abstract: To study the fuel atomization problem when raw vegetable oil is used as an alternative fuel of compression-ignition internal combustion engine, this paper investigated the differences in spray characteristics between raw sunflower oil and regular diesel under different experimental conditions using the Schlieren method. The focus of this study is the effects of fuel temperature and injection pressure on the spray penetration distance, the spray cone angle, and the air entrainment of fuels injected into the constant volume vessel. The results show that both the spray penetration distance and the spray cone angle of the sunflower oil increase with increasing the fuel injection pressure and fuel temperature, which are similar to those of regular diesel. When the fuel temperature is increased from 60℃ to 90℃, the average increases in the spray penetration distance and cone angle of the sunflower oil are 12.4% and 9.6%, respectively, at 60 MPa injection pressure. Such increases are 5% and 4.14% respectively at 120 MPa injection pressure. When the injection pressure is increased from 60 MPa to 120 MPa, the percentage increases in the mean spray penetration distance and cone angle are 39.7% and 16.7% respectively at the fuel temperature of 60℃ and 30.6% and 10.9% respectively at 90℃. When the injection pressure, background pressure, and fuel temperature are the same, the sunflower oil spray has a greater penetration distance but a smaller spray angle than that of diesel. The average increase of the spray penetration of the sunflower oil is 22.5% and the average decrease of spay cone angle is 60.3%. Elevating the fuel temperature or increasing the fuel injecting pressure can enhance the entrainment of air into the spray; However, the air entrainment of the sunflower oil spray is significantly weaker than that in the ordinary diesel spray. The results provide a valuable reference on the spray characteristics to help evaluate the feasibility of using raw vegetable oil as an alternative fuel for internal combustion engines.