Abstract: It is evident that the rainfalls has a remarkable effect on slope stability, but it is generally analyzed by static pore water pressure theory based on saturated soil. Actually, in most cases, the soil, especially the coarse soil and rocks, is discovered to be unsaturated or has a process from unsaturated to saturated state during rainfall. Therefore, a new slope stability analysis approach is necessary based on unsaturated soil theory. In this paper, the effect of unsaturated soils suction on slope stability is discussed from basic Mohr-Coulombs shear strength model. Based on that we suggest a modified GLE method considering suction for unsaturated slope. This method was used to an open pit mine slope and it revealed some basic laws of suction on slope stability. The slope safety factor from this new GLE method is higher than that from traditional one, but it has a bigger decrease in the process from saturated to unsaturated state and the final safety factor when the seepage being over is smaller than that from general pore water pressure theory.