The purpose of this study is to investigate the cause and mechanism of the debris flow at Tungmen. This preliminary study aims mainly on the investigation of geology and topography of the site as well as the material properties. The results will be very important for model tests in the future. The methods used in this study including site investigation, sampling, photography study, and large direct shear test. Also, by testing the materials with adjusted grain size distribution, the effect on the material properties can be understood.