An extended magnetohydrodynamic model including the effect of runaway current is developed and implemented in the M3D code. Based on this model, a simulation has been carried out to investigate the linear stability and the nonlinear evolution of the n = 1 resistive kink mode in the presence of runaway current. It is found that sawteeth oscillation is suppressed in a runaway plasma. The nonlinear evolution of the n = 1 mode only leads to a single sawtooth crash before reaching a new steady state axi-symmetric equilibrium with flattened current profile in the plasma core and q(0)>1.