The trail system inside the Yushan National Park is often damaged because of the slope failure induced by heavy rainfalls or earthquakes. The slope failure may threaten the safety of mountain climbers. This project conducted site investigation in the northwestern area of the Yushan National Park to identify the potential hazards, failure types and causes of the slope failures adjacent to the trail system. According to the results of field investigations, the major types of slope failure in this region are rock debris flow, rock fall and rock slide. Among all the cases of slope-failure sites, about 60% are debris flow, 12% are rock fall and 26% are debris flow combined with rock fall. The major reasons for slope failures can be attributed to the well developed foliation in metamorphic rocks, the heavily jointed rock mass and the intense weathering in the surface rock. Hazard-prevention maps were produced in this project. The potential hazard of the trail system is classified into two levels: “dangerous” and “with caution”, and marked by different colors. Besides the marking of potential hazard paths, the map also shows the locations for fallback positions and other relevent information for the prevention and avoidance of disasters. The project also looked into the retreat rate of the Chin-men-tong Cliff. Using the orthoimages produced from 5-phased sets of aerial photos (taken in 1977, 1990, 2000, 2007 and 2010), we concluded that the average annual retreat rate of this cliff over the last three decades or so was 5.3 m/yr heading the southwestern direction steadily. The unceased cliff reteat may threaten the trail system connecting Ba-Tong-Guang and the Yushan Peak. Moreover, stream capture may take place within one or two century: the Chen-Yo-Lan River may capture the upstream of the Lau-Ron River eventually. The trial section connecting Guang-Gau and Ba-Tong-Guang intersects several gullies; these gullies were intensely widened, deepened and extended upward during the Typhoon Morakot. As a result, the trail was forced to detour to the new route because of serious damage of the old trail route. The gullies may still expand in size and threaten the stability of the new trial route.