The fretting characteristics of the nitrile rubber O-ring for a mechanical seal were successfully simulated by axisymmetric finite element analysis, using the MooneyRivlin model with highorder terms. The effects of the displacement amplitude, medium pressure, precompression ratio and friction coefficient were investigated. The results show that three contact states for the fretting characteristics of rubber O-ring can be determined, i.e. stick state, mixed stickslip state and slip state with the increase of the displacement amplitude．The fretting running behavior of rubber Oring was strongly dependent upon the medium pressure, precompression ratio and friction coefficient. In mixed stick-slip state, the strong fluctuations of frictional force on the sealing interface would weaken the floatability of floating ring. In slip state, the high Von Mises stress may lead to shear failure and severe surface wear. However, Oring possessed the optimum comprehensive performance when working in stick condition. In a word, to mitigate shear failure and surface wear of Oring and meet the floatability and tracking property of the seal ring, the mixed stickslip state should be avoided and a compression of appoximately 10% and a lower coefficient would be preferred.