Qutie frequent slope failures have been occurred not only in natural slopes but also in engieered slopes. Although engineered slopes such as excavated and compacted slopes were carefully designed and well constructed, unexpected rainfall storms cause many slopes to failure. Nevertheless, we have to do our best to properly consider the real situations in the design. The state of slope safety changes as the rainfall infiltrates into the slope. It induces the pore-water pressure to increase. At present, there is no standard slope design methodology to properly consider the effects of rainfall infiltration. In the current specification of slope design, the surface water-table condition is suggested to prevent the slope failures in case of rainfall condition. However, the surface water-table condition is not relevant to present the real situation, in which only near the surface of slope becomes saturated unless the slope is highly permeable and has initially high water-table. Moreover improper saturated soil properties are generally used in the slope design process. Therefore the design results do not assure the slope stability during or after the rainfall. In this paper, an alternative slope design methodology is suggested to consider the real infiltration condition. It combines not only the local rainfall characteristics but also local permeability of slopes. This suggested slope design methodology includes the main factors affecting the slope stability such as; (1) reasonable initial profile of matric suction, (2) soil-water characteristic curve (SWCC) and permeability function, (3) prolonged rainfall condition, (4) nonlinear shear strength criterion, (5) shallow or deep (circular) slope failure mode, etc.