Convectional flow within the porous material of various models has received extensive attention in the last years.This interest is because of its wide range of applications, for example, in high performance insulation of buildings, chemical catalytic reactors, packed sphere beds, grain storage and such geophysical problems as the frost heave. In this study a numerical simulation of unsteady natural convection modes in a square enclosure filled with a porous medium having different wall thickness(0.2,0.3,and 0.5) has been done .It has been taken Darcy number effect (10-3,10-4,10-5and 10-6), with thermal conductivity ratio (1,5,10)these variables have been studied on the stream velocity and temperature effect for two cases of different heating ways one of them all down plate heated and the other for partial heating .The results show that increased in