Abstract: An analytical method is presented for predicting the buckling strength of the web-post in a castellated steel beam with the newly proposed fillet corner openings. Castellated steel beams with the proposed opening shape have the same good structural performance as those having circular openings. However, they are simple and economical to manufacture and there is no loss in the expansion ratio. The buckling modes and buckling strength of the web-post were studied using the finite element method. The parameters studied included the ratio of the opening distance to the opening height, and the ratio of the opening height to the web thickness. A strut model is proposed to predicate the buckling strength of the web-post in a castellated steel beam with the new fillet corner openings.