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Rainfall is one of the main factors that affect the slope stability. Rainfall infiltration will change the degree of saturation of soil, and further influence the pore water pressure and the permeability coefficient. One landslide occurred in Kurdestan province of Iran in 2009. The main reason of this landslide was assumed to be ground water flow and rainfall infiltration. This paper investigated accuracy of this assumption and predicted the effects of the horizontal drains on the ground water level and stability of case study slope during rainfall using twodimensional finite element analysis of transient water flow through unsaturated-saturated soil. The slope stability is evaluated by the global safety factor, based on the two-dimensional limit-equilibrium analyses method. The results show that the rainfall is one the main reason of landslide. Also analysis of drains performance show that the horizontal drains can effectively lower the ground water level and consequently increase the slope stability during rainfall.