Reduction of angle of internal friction with increasing stress level of sandy soil is widely recognized and noticed by many researchers. There are limited studies on the effect of this reduction on bearing capacity of piles. This book presents a study of the effect of this reduction on axially loaded bored piles embedded in dense and medium sand by using finite element approach. The soil modeled using a hyperbolic soil model with an empirical equation to account for the reduction of angle of internal friction with increasing stress level while the concrete bored pile modeled as a linear elastic material. Investigating the effect of the stress level on behavior of piles, bearing capacity factor Nq, and distribution of shear stresses in soil-pile interface along the pile shaft were made during this work by pass through a wide range of stresses by using piles with dimensions starting from laboratory dimensions and goes towards field dimensions with an embedment ratio L/D range (15, 20, 25, 30, and 40). Also, two models to estimate pile capacity in medium and dense sand are developed in this work using statistical techniques based on finite element analysis results.