Abstract: Concavity on vertical inner floating roof tank is the consequence of tropical storms in summer season. This phenomenon may be caused by two reasons; (1) higher wind pressure of the tropical storm than the design wind pressure of the vertical tank; and (2) negative pressure resulting from rapid decrease of ambient temperature and insufficient ventilation volume in breather valve. By means of ANSYS mechanics reasons for tank concavity are analyzed through finite element (FE) model and a conclusion is that valve distortion resulting from the impact of wind pressure is very small. Also ventilation volumes in breather valve are calculated, and in consequence, a major cause of the accident is lacking of sufficient ventilation volume in breather valve. Suggestions are given for improving the safety of the vertical tank.