||Effects of cracks and rainfall on slope stability are discussed in this work. A two-dimensional model of a hypothetical cracked slope is developed for parametric study of slope stability, in which cracks are modeled as vertical openings to the slope surface. Two programs "Seep/W" and "Slope/W" are employed for calculation of slope safety factors. Pore water pressure distributions provided by "Seep/W" are used in "Slope/W" for subsequent stability analysis of the slope. A series of parametric studies with varying crack depths and spacing, rainfall amount and pattern has been conducted, to determine their effects on pore water pressure distribution and slope stability. The results show that stability decreases with deeper and/or denser cracks. Moreover, the presence of surface cracks tends to produce shallow critical slip surface, provided that the slope contains a large enough number of cracks. It is also interesting to find that the slope containing random cracks is safer than that with identical cracks. Heavier rainfall leads to less stability. Even with the same amount of rainfall, heavier rainfall in the early part of a storm tends to be more dangerous. To determine the uncertainty of stability due to inherent variation or prediction error of input parameters, a probabilistic approach is adopted for stability analysis. Uncertainties of rainfall amount, crack depth, crack spacing, and soil shear strength are estimated and the FOSM (first-order second-moment) method is used for reliability evaluation. The failure probability of a hypothetical slope is estimated to be 0.10. The effectiveness of several slope improvement programs is also studied. Regular maintenance and inspection of cracks turns out to be an effective way to improve stability.