A theoretical model describing apparent viscositv of gas-solid fluidizing particles for spherical particles in Couette flow was proposed, and the following results were obtained. 1) Estimated values of apparent viscosity as functions of Young modulus and density of particle, Coulomb's friction factor and some adjustable parameters agreed well with the trend of earlier experimental data. 2) In the case of bimodal particle size distribution, the observed values apparent viscosity are expected to be markedly smaller than the estimated ones. 3) The proposed model supposes that rheological behavior exhibits dilatant rather than Newtonian fluid because apparent viscosity increases with particle velocity in the case of spherical particles with smooth surface in atomic and/or molecular size level.