A 2D Discrete Element Model was employed to simulate the rupture propagation and near-source ground motion in the epicentral area of the 1999 Chi-Chi (Taiwan) earthquake. The observations show that the hanging wall side is characterized by larger particle motions than the footwall side. The simulation results reproduce the main features of the recorded ground motion and show that the particle velocity on the hanging wall side and on the footwall side are symmetric in the deeper fault region, however, as the crack approaches the free surface, the hanging wall side is characterized by larger particle motions than the footwall side. These results suggest that the difference in the particle motion on the hanging wall and on the footwall is due mainly to the asymmetric geometry of hanging wall and footwall, Thus, the model used leads to good approximations in the vicinity of the epicenter.