Approximate formula of stability lever ( GZ ) was studied theoretically for the ship with supertructures. The lever ( GZ ) is divided into two parts; one of under deck volume ( GZ ₀) and another due to superstructures (δ GZ ). The latter was expressed as the third degree equation of heeling angle and it can be easily calculated using the particulars given at the early stage of design. So the lever GZ is given as the sum of δ GZ and GZ ₀ which can be calculated by the known formula. Also the value a, b which are very important factors for the stability formula was calculated very accurately by the theoretical consideration of so called effective freeboard.