The critical conditions to mark the onset of natural convection in the horizontally superposed fluid and porous layer, cooled from below, with variable porosity were investigated. By using the linear stability theory the effects of d_m/d_p(ratio of porous layer thickness to particle diameter) and ^d(ratio of porous layer thickness to fluid layer thickness) on the critical conditions are obtained numerically. The results show that the critical conditions are influenced by the variation of porosity when the value of d_m/d_p is smaller than 10³. For large values of ^d the onset of convection is dominant in the porous layer and the critical Darcy-Rayleigh number decreases with decreasing the vale of d_m/d_p. It means that as the particle diameter becomes larger, the porosity becomes larger near the surface and the fluid inthe porous layer becomes more mobile, i.e., more unstable.