We present in this paper a numerical study of sandpile formation and angle of repose by considering the effect of particle shape. We analyze the micromechanical responses of sandpiles, which are crucial to the exploration of origin of angle of repose. The results show that the principal anisotropy directions of contact orientations for the left and right parts of sandpiles deviate increasingly away from the vertical direction as the particle shape becomes more irregular, and that the summation of their deviation angle relative to the vertical direction with angle of repose , is approximately a constant regardless of the effect of particle shape. We find that the principal anisotropy directions of particle orientations for the left and right parts of sandpiles rotate as the irregularity of particle shape varies, and tend to reach a compromise state and to lie in the common horizontal direction at a characteristic aspect ratio AR = 0.6. We reveal that the mobilization of arching effect depends primarily on the inclined propagation of strong force chains. We also establish a relationship between the direction where the most intense arching phenomenon takes place and the principal anisotropy directions characterizing the distribution of microstructures and inter-particle force network.