The stability of ion shell distribution is an interesting and important issue in view of the fact that it is stable under perturbations propagating parallel to the ambient magnetic field, despite large amount of free energy it contains [Freund and Wu, 1988; Brinca, 1990>. It was found, however, that if the shell distribution of ions has a sufficient amount of drift speed with respect to the solar wind hydromagnetic waves, it can become unstable [Gary and Sinha, 1989; Yoon et al., 1989>. In addition, recently Wu and Yoon [1990> have found the obliquely propagating hydromagnetic instability associated with the ion shell distribution. This letter presents the unified stability analysis of the ion shell distribution, and show under what condition the drift-induced instability of the type first discussed by Gary and Sinha [1989>, and the oblique hydromagnetic instability discussed by Wu and Yoon [1990> are excited and how the two types of instabilities are related. ¿ American Geophysical Union 1990