The characteristics of flow and particle diameters in the unsteady spray from an air-assisted fuel injector in atmospheric conditions have been analyzed using a phase Doppler anemometer (PDA). The frequency of injection was 50 Hz, and the pressures of both fuel and air were changed in three steps. Radial and axial distributions of bulk velocity and Sauter mean diameter of spray particles were obtained. It is found from the experiments that the profile of mean velocity is closely related to the normal distribution and the Sauter mean diameter of droplets in the spray is as small as 20 to 30μm. And the log-hyperbolic function (LHF3) has also been applied to the spray. Consequently, the characteristics of the distribution of spray particle sizes are represented clearly by the three parameters of the LHF3.