The effects of gas velocity, solid circulation rate, the secondary air injection ratio, and secondary injection type on the radial gas-mixing coefficient (D-r) in a circulating fluidized bed (CFB) (0.1 m i.d., 5.3 m high) have been determined. It increases with increasing solid circulating rate and the secondary air ratio (SAR), but decreases with increasing gas velocity. Based on the present and previous studies, D-r increases with increasing column diameter. The effect of secondary air injection type on the radial gas-mixing coefficient is more pronounced with the tangential than with the radial air injection. The radial gas-mixing coefficient in terms of Peclet number has been correlated based on the isotropic turbulence theory.