Hydrogenated nitrile-butadiene rubber (H-NBR) is a high-performance elastomer with important applications in the automobile industry and petroleum field. Selectively catalytic hydrogenation of nitrile-butadiene rubber (NBR) with a typical olefin metathesis catalyst, Grubbs II, has been developed in the present work. The catalytic selectivity for the hydrogenation of -C=C- instead of -C≡N was supported by FT-IR and NMR analysis. The effects of the reaction conditions, such as reaction time, temperature, pressure, catalyst amount, and solvent, on the hydrogenation degree (HD) and relative molecular weight and distribution of the hydrogenated products have been investigated in detail. Under the optimized condition, 6 mg catalyst per 1.0 g NBR in 100 mL toluene solution and H2 pressure of 5.0 MPa at 55 °C for 3 h, a hydrogenation degree of 75% was obtained. In the case, the degradation of polymer chains might provide a solution for the challenge of high Mooney viscosity of H NBR, strongly restricting its processability in many applications.