Dispersion of aggregate particles by acceleration in an air stream is examined both experimentally and theoretically. In the experiments, several kinds of powders composed of particles of around 1 mu m are dispersed by a high speed air stream at the throat of an ejector type disperser. The dispersed particles are observed with an optical or electron microscope. A numerical simulation is made with aggregates of different sized spheres, comparing the dispersion force acting on each aggregate with the adhesion force between spherical particles. The experimental dispersion results obtained for some kinds of powders agree with the simulation results. In the case of a few other kinds of powders, dispersion of the aggregate particles is actually more difficult than suggested by the calculations based on the assumption of spherical primary particles. Comparison between the experimental results using an ejector type disperser and the numerical simulation results is useful for the evaluation of adhesion between particles.