In slope stabilization work, a reinforced soil method using micro-piles in vertical direction enables space-saving construction. While some slope stabilization works have utilized vertical micro-piles, it has been pointed out the likelihood of soil loosening (due to the movement of soil mass or erosion) between the micro-piles increases. This paper provides the mechanical overview focusing on the pile spacing and the arrangement of piles in the view point of soil loosening by three-dimensional discrete element method (3D-DEM). The small arrangement of the micro-piles showed that the normal particle force was relatively larger compressive condition comparing to the case of coarse arrangement of micropiles. variations of the behavior of soil loosening is factor on pile arrangement.