The nonlinear dynamics and the vibration isolation effectiveness of a two-degree-of-freedom nonlinear vibration isolation system are numerically studied. The complex nonlinear behavior in the force-frequency plane of the system is analyzed. Cascades of bifurcation of the system with different excitation amplitude are also obtained. The power flow transmissibility is analyzed to validate the performance of the system in vibration isolation. The numerical results show that the reduction of the line spectra when the system is chaotic is much greater than that when the system is nonchaotic, and that the overall effectiveness of vibration isolation at chaos is better than that at nonchaos.