In a downer reactor (0.1 m-I.D.×3.5 m-high), the effects of gas velocity (1.6-4.5 m/s), solids circulation rate (0-40 kg/m²s) and particle size (84,164 ㎛) on the gas mixing coefficient have been determined. The radial dispersion coefficient (D_γ) decreases and the radial Peclet number (Pe_γ) increases as gas velocity increases. At lower gas velocities, D_r in the bed of particles is lower than that of gas flow only, but the reverse trend is observed at higher gas velocities. Gas mixing in the reactor of smaller particle size varies significantly with gas velocity, whereas gas mixing varies smoothly in the reactor of larger particle size. At lower gas velocities, D_γ increases with increasing solids circulation rate (G_s), however, D_γ decreases with increasing G_s at higher gas velocities. Based on the obtained D_γ values, the downer reactor is found to be a good gas-solids contacting reactor having good radial gas mixing.