The objective of this work is to study the effect of the relative densities of the components in fluid-solid mixture using the formulation presented in Johnson et at., (1991 a,b). The mixture theory is modified to include the effect of buoyancy forces; for this purpose we introduce a term, A(6) v gradp, in the mechanical interaction between mixture components. When coefficient Ag is zero, We recover the results presented in Johnson et al., (1991 a,b). It is expected to see settling of the solid particles toward the lower plate which is normally related to the "buoyancy effect" in solid-fluid mixtures. Equations for two-component flows are used to analyze Poiseuille flow between two parallel plates. A review of the basic principles of the mixture theory is presented. The equations for the stress tensor of each component of the mixture and the interactions between the components are given. Flow of a fluid-solid mixture between plates is presented with numerical methods and results.