[en] Free boundary MHD equilibrium properties of Heliotron J plasmas are investigated by the VMEC code and the PIES code, and ideal MHD stability properties are studied by the Mercier criterion, the ballooning mode equation and the CAS3D global stability code. Since a plasma in the standard configuration of the device has relatively small rotational transform (∼0.55) and weak magnetic shear and its effective toroidicity is comparable to the inverse aspect ratio, difference of the rotational transform profile in the fixed boundary equilibrium and in the free boundary equilibrium is not small even in a low beta plasma because of the Shafranov shift of an entire plasma column. In the free boundary equilibrium obtained by the PIES code, magnetic islands corresponding to the ι/2 π = 4/7 resonance appear above β₀ ∼ 1%. In the standard configuration, the local analysis by the Mercier criterion and the ballooning mode equation shows that the ballooning mode can be unstable even in a low beta plasma, though the interchange mode is stable up to the numerical equilibrium beta limit. The relation between the local analysis by the ballooning equation and the global analysis by the CAS3D code and the properties of the ballooning mode in a Heliotron J plasma are discussed. Copyright (2002) Australian National University- Research School of Physical Sciences and Engineering