Abstract. Nonlinear MHD simulations on relaxation phenomena in spherical tokamak and helical plasmas, including 3D HINT equilibrium computations, are reported. 1. The dynamics of global relaxation phenomena in spherical tokamak have been revealed. We analyze the sawtooth and IRE. For the sawtooth case, a new nonlinear triger mechanism is found, in which the n=1 activity in the core region is subsequently excited by development of higher n ballooning modes in the periphery region. The overall shape of the torus surface does not change throughout the event. On the other hand, our previous simulations for IRE show a large distortion of the torus. The differences arise from only slight discrepancies in the initial profiles. 2. The 3D equilibrium code HINT, which does not assume the existence of magnetic surfaces, is modified in a couple of directions to extend the functions. Based on a HINT equilibrium, nonlinear evolution of instabilities is studied in a full 3D geometry of LHD. An initial phase of plasma deformations is dominated by medium-n resistive ballooning instability. In the later stage, a well-confined state is recovered by having spontaneous evolution to a broader pressure profile.