An impulse injection phosphor tracer technique is proposed to study the effect of particle properties, including particle size, particle density and particle sphericity, on the lateral mixing behavior in a riser with FCC particles as fluidized materials. The RTD curves of all kinds of particles have one peak, which can be described by a two-dimensional dispersion model. Between 15 and 80 mum, the lateral solids dispersion coefficient may have a maximum value, indicating that there exists a minimum particle size at which the particles do not aggregate intensively in the riser. The lateral solids dispersion coefficient decreases with increasing particle density and solids concentration, while increasing with increasing particle sphericity and superficial gas velocity. A correlation is developed to account for the influence of particle properties, operating conditions on the lateral dispersion coefficient, which fits the experimental data well.