Radial gas mixing in a fluidized bed was studied using response surface methodology (RSM), which enables effect examinations of parameters with a moderate number of experiments. All experiments were conducted in a 0.29-m ID fluidized-bed cold model. The gas dispersion process within the bed is described using the dispersed plug flow model. Pure carbon dioxide was used as the tracer gas, continuously injected into the center of the bed by a point source. The downstream radial tracer concentration profile was measured using a gas chromatograph. The radial gas dispersion coefficient, D-r, was well correlated with operating parameters and the particle and gas properties: (U - U-mf)/U-mf, H-s/d(b), phi(d), and Ar, with a determination coefficient R-2 of 0.966. Effect test indicates that the dimensionless characteristic velocity, (U - U-mf)/ U-mf, has the most significant influence on D-r, while the static bed height to bed diameter ratio, H-s/d(b), is less remarkable. The interactions of (U - U-mf)/U-mf with the distributor open-area ratio, phi(d), and with the Archimedes number, Ar, both play important roles. An evolutive response surface model was proposed to describe the radial gas mixing in the bubbling/slugging fluidization regimes.