Dynamic analysis of a three-axle heavy truck is carried out with rigid body model and flexible body model. To see the effects of chassis flexibility, the chassis is modeled as flexible body. The mass matrix, stiffness matrix, and vibration normal modes of the chassis are obtained by a finite element analysis program, and four vibration normal modes are used in the flexible body model. The vehicle model consisting of a frame, a cab, suspensions, an engine, a deck, a seat, and tires, has total 77 degrees of freedom. The result shows that the peaked acceleration in the flexible model is lower than that of the rigid body model.