Abstract: The web-post buckling behaviors and shear buckling strength of castellated steel beams with fillet corner hexagonal openings at room temperature and in a fire were investigated through the strut model and finite element simulation. S/d₀ and d₀/t_w varied from 1.4 to 2.0 and from 41.4 to 80.7, respectively. Based on the parameter studies, modifications were made to the effective width of the strut and the compression force acting on the strut. And an equivalent rectangular theory was proposed for calculating the shear yielding strength at the perforated section. The shear strength of the castellated steel beam took the minimum value of the shear buckling strength of the web-post and the shear yielding strength at the perforated section. The shear strength calculated by the modified strut model and the equivalent rectangular theory was only a little smaller than that calculated by the finite element method. The ratios between them varied from 0.856 to 0.978. The web-post buckling temperature calculated through the modified strut model and BS5950 was only a little lower than that obtained through finite element analysis. The differences between them were only 0.17%~6.28%.