Recent theoretical results of MHD instabilities in Heliotron/Torsatron are reviewed. By comparing the results with experimental data in Heliotron E, Heliotron DR and ATF, it is pointed out that resistive interchange modes are the most crucial instabilities, since the magnetic hill occupies a substantial region of plasma column.
Development of three-dimensional MHD equilibrium codes has made a significant progress. By using the local stability criterions shown by D_I (ideal MHD mode) and D_R (resistive MHD mode) for the equilibria given by the three-dimensional codes such as BETA and VMEC, stability thresholds for the low n ideal modes or the low n resistive modes are estimated, where n is a toroidal mode number.