A segregation velocity model is presented here to describe the segregation behaviour of the fluidized beds containing binary mixture of particles differing both in their size as well as the density. Using the Richardson-Zaki correlation, the model evaluates the segregation velocity of the larger particle species which is assumed to be present in the pseudo fluid consisting of a homogeneous mixture of the liquid and the smaller particle species. The present model is capable of correctly describing the particle motion responsible for the observed segregation tendencies of the fluidized bed. Moreover, the model predictions of the zero segregation velocity compares very well with the experimental data of the layer inversion phenomenon reported in the literature. The model tends to over-predict the inversion velocity for cases where the diameter ratios of the larger to the smaller particle species are less than 2.