were large landslides and rock falls in the debris flow gully in the earthquake area during Wen- chuan Earthquake. The initiation condition of debris flow was changed greatly. To find out the critical velocity of flood for initiating debris flow, experiments of critical hydraulic conditions for initiating particles and debris flows were conducted in the laboratory. The conclusions are that the critical condition of initiation of debris flow is between the conditions of initiation of a little particle and abundance particle. The probability of critical initi- ation is 0. 05, two times as much as the probability of initiation of a little particle, one third of initiation proba- bility of abundance particle. The critical velocity of initiation of particle and debris flow increases with the me- dian particle size and asymmetrical coefficient of sediment. It decreases with the bed slope. The revised equa- tion of initiating velocity of particle and debris flow can be used in the calculation of initiating velocity of coarse particle and large asymmetrical coefficient of sediment.