Collapse due to buckling is a physical phenomenon which occurs severely under compression or tension. Plates having small thickness with respect to other dimensions, are more susceptible to buckle. When plates containing initial defects such as cracks, load bearing capacity of the plate decreases and buckling will occur under less loading. In order to increase the strength of the structure and its load bearing capacity, investigation on buckling of plates, especially those with crack defect, becomes more necessary. Thus destructive effects of buckling can be prevented. In this study, buckling behavior of stiffened and unstiffened Aluminum 1100 alloy plates which including cracks have been investigated by using the linear finite element method program ABAQUS. Parameters such as crack length, crack orientation, crack location on the plate, boundary conditions and stiffener effect have been considered. It has been found that existence of crack can decrease the buckling load. Also by using stiffeners, the buckling load of the plate increases to a great extent. As a result, existence of crack decreases buckling load of unstiffened plates to 28 percent in the most critical case, and it decreases buckling load of stiffened plate to 17 percent in the most critical case.