Various investigated methods of aerial photographs, geomor-phological characteristics, geological condition, geomaterial properties and stability analysis were used to identify these hazardous mechanism by two debris flow cases in Tungmen, Hualien and Shing-Yi, Nantou. The investigation results demonstrated that these deposited materials were triggered and moved downward to the lower part of the gully by high intensity precipitation. These geomaterials coverged from the sidewall of the gully by cutting and erosion from the moving debris flow.