In this study, entropy generation in laminar mixed convection in a square fluid filled cavity is numerically studied. The middle of the lower wall of the cavity is heated to a constant temperature Th while the side walls are maintained at a constant temperature Tc, and moving upper ward at a constant velocity to understand the effects of irreversibility distribution on the entropy generation for different engineering applications. For the studied control surface, remaining parts of lower and upper walls are adiabatic. The finite difference method is used to solve the governing equation. The entropy analysis is carried out to determine the irreversibility which is generated in the cavity for different Prandtl number (Pr=0.0212, 0.71 and 6.35), the effect of the irreversibility distribution ratio on entropy generation was investigated. It seen that effect of irreversibility distribution ratio (γ=10 2 and 10 4) have play important role on the total entropy for different Prandtl number. Also it is clear that, for all Prandtl number, the total entropy generation increase by increasing the irreversibility distribution ratio and the increase of Prandtl number regardless the values of Ri=1 and Re=100 because of the increase of the velocity gradients.