A numerical analysis of nonlinear behavior of m=1 resistive internal kink instability in a Tokamak is presented in a cylindrical approximation. A two-dimensional nonlinear code which solves incompressible resistive MHD equations with helical symmetry has been developed. For the case of small longitudinal wave number k, the magnetic axis moves outward until it reaches the critical surface where the value of helical magnetic flux has the same value as the one at the magnetic axis. On the other hand, as k is increased, the shift of the magnetic axis saturates before touching this critical surface. This result gives a possible explanation of suppression of internal disruptions observed in high power neutral beam injection experiment to Tokamaks.