ABSTRACT. The morphological change of fine particles induced by impaction against a solid surface has been investigated for two types of tristearin fine particles which have different mass median diameters. It is found that the bulk density of the heap formed on the solid surface increases with increasing the impaction velocity and the mass median diameter of the tested powder. The change in the bulk density can be evaluated by the effective impaction load which is calculated by the theory of elasticity, considering the particle size distribution and the mechanical properties of particles. The heap formed on the solid surface at low impaction load consists of fine particles, while at high impaction load, it consists of a caked heap where no particles are found. AEROSOL SCIENCE AND TECHNOLOGY 26:343-3% (1997 compression load (N) effective impaction load applied on a single particle (N) mass of the heap (mg) mass of particle (kg) coefficient of uniformity of breaking stress distribution ( -) compressive breaking stress (Pa) clean sheath air flow rate (m3/s) aerosol flow rate (m3/s) mixing ratio (-) air velocity at impactor jet (m/s) u impaction velocity (m/s) V particle volume (m3) E; Young's modulus of particle (Pa) a flow rate ratio ( = Q,/Qp) (-) vi Poisson's ratio of particle (-) pH bulk density of the heap (kg/m3)