Abstract. Free boundary MHD equilibrium properties of Heliotron J are investigated by VMEC, HINT and PIES codes, and ideal MHD stability properties are studied by the Mercier criterion, the ballooning mode equation and CAS3D global stability code. It is shown by the equilibrium calculation that the change of plasma boundary shape is substantial in a low shear helical system even if the beta is relatively low. Comparison between PIES results and HINT results is shown. From the stability analysis, good correlation is found between local and global analyses for the 3D or helical ballooning mode whose mode structure shows strong poloidal and toroidal mode couplings. In the 3D ballooning mode, the eigenmode is localized in a flux tube. It is also found that the positive shear of the rotational transform is favorable for the 3D ballooning mode stability in a low shear helical system. The role of the magnetic shear on the 3D ballooning mode in a low shear helical system is clarified.