It is difficult to estimate reduced buckling length of struts which are main structural members in a wingtank of a ship, because they are tightly connected with the transverse girder and have variable cross sections at the ends. The author and others have suggested through numerical calculations^{7) '8)} that strut-end structures give a great influence upon the buckling strength of struts. In this paper, approximate calculation of reduced buckling length of struts under compression is suggested, in which strut-end structures are treated as a spring supporting face-bars. The results of the calculation show fine agreement with ones of FEM buckling calculation and suggest how much reduction of flexural rigidity of face-bars should be taken into account at round corner. It is also investigated how much restraining effect is given on buckling strength of struts by tripping brackets. The results show that tripping brackets are most effective against buckling of struts when they are straightly connected with face-bars at the ends of round corners and they lose the effect rapidly as the connected points are rotated along the round corner, consequently the boundary condition of face-bars is regarded as fixed at θ=0° (see Fig. 7) while it is regarded as simply supported at θ=90°.