Abstract: The truck train consisted of universal truck and semi-triler for the treelength transportation is equipped with carting beam.Itis very important to set the optimum location of carting beam, which would realize rated load of the front and the rear axles under condition of full load, static state and flat rood, thus making truck develop the highest efficiency. It is under the direction of said idea that this paper, considering the circumstances in which the technical eguipments for log transportation are installed on the trucks and the down-grade characteristics hauling direction on forest road in China, proposes by the principle of mechanics a formula with the help of which the optinum location of carring beam can be determined, and presents some examples of calculation.