An accurate estimation of the lateral force is an important key point for the stability analysis, because the lateral force influences both pile and slope stabilities. Therefore, overestimation of the lateral forces on the pile will naturally lead to conservative pile design and un-conservative slope stability design and vice versa. This is because the estimated lateral loads are used as extra resisting force(s) on slope stability calculations due to the pile reaction. In this paper, 2 different methods of lateral force calculations namely plastic deformation and visco-plastic flow are obtained on a pile that installed into slope which is susceptible to failure. First, the case of a slope without piles is considered and the methods are then used to calculate the safety factor for the slope. Then, the stability of slopes reinforced with piles is analyzed. Expressions are derived allowing the force needed to increase the safety factor to a desired value and the most suitable location of piles within the slope to be evaluated.