Abstract: The atomization characteristics of a plain orifice at subatmospheric pressure were studied by means of pulsed laser holography,and hologram images of cross-spray downstream fuel cloud field were analysed with computer image system.The results obtained show how the mean particle size and particle size space distribution vary with air velocity、ambient pressure and spray fuel pressure.The first,the air velocity is a primary factor that affacts the fuel atomization at either normal or subatmospheic pressure,the mean particle sizes monotonically increase as the air velocity decreases.The second,as the air density reduces with descending ambient pressure,aerodynamic force and turbulent fluctuation are reduced which force spray fuel sheet to breakup and drop size to fall to a critical value.As a result,the penetration is augmented,atomization is retarded,and mean drop diameter is increased,as well as fuel density space distrbution becomes non-umiform and the quality of atomization obviously deteriorates.Therefore,the ambient pressure is also important to atomization.The last,when the nozzle spary goes across the direction of air,at subatmospheric pressure spary penetration depth rises more rapidly than at atmospheric pressure as the fuel-injection pressure increases.