Abstract: Stability of equilibrium of several simple floating bodies and ship models was investigated, and the maximal allowed external torque for ship models was analyzed and calculated. The analysis and calculation procedure was generalized and applied to real ships of any shapes. This may help to improve ship design. 6 criteria for stable equilibrium of floating bodies were compared, and the advantage and limitation for each as well as their possible relations were pointed out. This helps to clarify some basic concepts and may benefit teaching and research in this field.