Buckling and collapse of the strut in the wingtank of ore carriers are investigated by model tests and finite element analysis. It is also discussed how loading conditions have effects on buckling and collapse load. The results are summarized as follows : (1) The deflection at the center of struts can take place even under small external load due to the one-sided stiffeners of web plates. As a result of this, the collapse load of strut is 4060% less than the buckling load calculated by elastic analysis. (2) The axial buckling load of strut with bending moment may be 2030% less than that without bending moment in the loading condition of the ship at sea.