This paper presents a procedure for calculating the safely factor for an unsaturated slope suffering from rainfall infiltration. The process of infiltration into a slope due to rainfall and its effect on soil slope behavior were examined using a two-dimensional finite element flow deformation coupled analysis program. The modified Mohr-Coulomb failure criterion was adopted for unsaturated soil strength to consider variations in strength due to the presence of matric suction. A safety factor was calculated based on the smoothed stress field obtained from finite element analysis, and an optimization technique was used to search for a critical slip surface. Some numerical examples are analyzed. The results illustrate a typical process of infiltration into unsaturated soil slopes and the effect of hydraulic conductivity on slope stability during rain-induced infiltration. (C) 2001 Elsevier Science Ltd. All rights reserved.