Debris flows are usually initiated on hillsides or in channels on the upstream of a watershed. Initiation criteria and dimension of debris flows are related with the amount of colluvial soilds, and geologic, topographic and hydrologic characteristics of hillsides where debris flows occur and travel down through. The relationships between the occurrance of debris flow and the topographic slope, watershed area and rainfall characteristics are reviewed in order to discuss the assessment and prediction of debris flow hazards. The potential travel distance of debris flow in a channel and the potential travel distance, spreading width, and spreading angle in an alluvial fan are also discussed.