With a high-order weighted compact nonlinear scheme(WCNS) for space discretization,several different implicit methods,including LU-SGS,Gauss-Seidel point-relaxation,line-relaxation and GMRES(generalized minimal residual),are compared in the simulation of hypersonic viscous flows.Both point and line relaxations with analytical Jacobian matrix converge faster than those of LU-SGS.GMRES obtains a faster convergence rate combined with line relaxation.