Piles have proven to be an effective means of stabilizing active landslides as well as in marginally stable slopes. Many practical empirical design and analysis methods of slope stability and piled-slope stability have been proposed. However, the solutions of analysis and design methods proposed vary due to different analysis methods used and the design methods are poorly understood because the pile-stabilized mechanism is complex. This thesis presents the results of two-dimensional finite element analyses with strength reduction method using the ABAQUS package to validate slope stability analyses of four cases, which are (1) homogeneous slope without foundation, (2) homogeneous slope with foundation, (3) non-homogeneous slope with foundation and (4) non-homogeneous slope with a thin weak layer. The results of unreinforced analyses are validated for each case based on limit equilibrium or finite element analyses in terms of factor of safety. With the results validated, a pile is incorporated in the model and analyzed using coupled analysis, which considers the slope stability and pile response simultaneously. Numerical analyses results based on pile position in the slope, pile head condition, and pile length are used to determine the optimal pile position, suitable pile head condition and appropriate pile length to increase the stability of pile-stabilized slopes. A three-dimensional finite element model of the slope stability is also conducted and the factor of safety found to be higher compared to the results of two-dimensional finite element analysis. The spacing effect of pile is examined in a three-dimensional piled slope model and the factor of safety is found to approach the case without pile when the ratio of spacing to pile diameter is equal or greater than 8.0. An optimal pile spacing of S/D of 4.0 is found. Based on study of the influencing factors in piled slope stability, an optimal design is proposed.