The particles in the riser are exerted by the Kutta-Joukowski transverse force and the concentration gradient force simultaneously in the radial direction. Thereby a great number of particles gather near the wall, as a result, a stable core-annulus structure occurs. Based on the experimental data, the radial distribution characteristics of the particle concentration gradient were analyzed. In addition, according to the proposed the relationship of the Kutta-Joukowski transverse force to the concentration gradient force, the s of the concentration gradient force, Fρ=K(dρ/dr)A, and that of the concentration gradient force coefficient K=[?ρg(νg?νp)(?v/?r)r]/[dρ/dr+(d2ρ/dr2)r] were presented. The concentration gradient force on the particles in riser was dependent on the concentration gradient force coefficient. Radial profiles of the concentration gradient force coefficient (K) in the riser was given and the influences of operating parameters were investigated. The results showed that the value of the concentration gradient force coefficient was 0 at riser center while the radial distribution assumed the N-profile. The concentration gradient force coefficient increased with the increasing of the superficial gas velocity. The value of the coefficient in the fully developed zone of the riser was significantly higher than that in the acceleration zone as well as the outlet constraint zone. Based on the analysis results, the empirical correlation of K in riser was established.