The Nan-shih-keng landslide is located at the Tainan Highway No. 174, which is a disaster prevention route connecting the Tsengwen and the Wushantou dams at south Taiwan. Although the authority built a retaining wall and cables as hardware countermeasure to stabilize the slope, abnormal ground movements continued. To ensure the security of the local residents and highway, and the possible generation of the landslide dam, evaluating the impact area of the landslide is essential in the soft countermeasure. This study applies an alternative approach to assess the impact area of the landslide by a three-dimensional (3D) discrete element method (DEM). The 3D DEM is a dynamic numerical method, which explicitly considers the geometry of topography and joints, block contacts, and large block displacements with solid physical and mathematical theory. In this study, each block is assumed to be rigid. The retaining wall and the cables are assumed functionless. Three joint friction angles, ?=10°, 15°, and 18° are applied to the landslide simulations. The computational results indicate that the rocks near the highway have the lowest stability. Block movements are limited to the ones near the highway when the ?=15° and 18°. When the ?=10°, extensive slope movements generate a landslide dam near the slope toe with the length of 465 m. The deposit thickness at the creek is 20m. The sliding rocks climb the opposite slope with the horizontal distance of 108 m. The monitoring block near the highway has the runout distance of approximately 420 m and has the maximum block velocity of 29 m/sec. The computational results indicate that without considering the function of retaining wall and the cables, the significant landslide may occur during the heavy rainfall. In addition, the 3DEC is a useful tool showing both the movements of the jointed rock mass and the 3D impact area for the authority to manage the land use and to suggest corresponding disaster mitigation countermeasures.