Abstract: A 2-DOF nonlinear dynamic model was developed with a differential equation of motion involving cubic nonlinear stiffness given for motor vehicles and the amplitude-frequency response was solved by the method of multiple scales. A numerical simulation was done to reveal the amplitude-frequency response of the nonlinear suspension system involving different nonlinear parameters when there is a simultaneous coexistence including the main, superharmonic, subharmonic, and internal combination resonance. The results showed that the nonlinear parameters should be chosen reasonably so as to get rid of the possible instabilities of suspension system such as skipping and bifurcation, thus controlling efficiently the vibration of motor vehicles.