The effect of dry-wet history on the erosion process of cohesive sediment was investigated experimentally in the present study. Detailed discussion on the mechanism of this process was conducted here on the basis of enormous number of experimental data. In this experiment, the test sample keeps under dry or wet conditions every 6 hours, and erosion test was conducted just after each period. It wasfound that this process can be classified into two parts in their mechanisms according to the total period of this history. The erosion rate of test sample which experienced this history more than one day keeps almost constant value which agrees well with the predicted value of the erosion rate formula.