Abstract: An elastic model by hypothesizing that the two heads respectively contain different elastic elements during kinesin chemomechanical cycle is proposed, and according to the Bohzmann law we obtain the exact expression for the ratio of theprobability of forward step to that of backward step in the presence of load force, which provides a physical explain for the data of Carter and Cross.

Key words: kinesin, elastic model, ratio of forward step probability to backward step probability