Abstract: The sediment incipient motion can be significantly affected by the presence of microbial biofilm growth.Using the experimental incipient velocity data collected under the influence of biofilm colonization,the incipient velocity formulas for the two types of sediments are derived for sliding and rolling motions.The thin water film theory is utilized to describe the coheslve force between sediment partmles and the aClheslve force generated by the biofilm in the formula derivation.Time variation characteristics of the biofilm strength and the feature of the substrate are also taken into the consideration.The result shows that both rolling and sliding formula are able to reproduce the observed incipient Veloci1t1es.The two calculated velocities are basically consistent with each other.