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 relevant information for the prevention and avoidance of disasters.