The impact of number of opening and spacing between the openings for mild steel cellular beam is discussed in this article. This work was carried out by comparing the buckling behaviour of castellated beams in a series of experimental works and nonlinear finite-element (FE) modeling. Finite-element analysis was used to analyse the failure load, failure displacement, load–displacement curve, and failure mechanisms of cellular beam. Furthermore, the strength and buckling behaviour of cellular steel beams were studied parametrically using a finite-element model to examine the effects of changes in cross-sectional geometry, opening size and number, separation between openings, and end post distance. The results show that simply supported cellular steel beam with a point load at the center and S/D₀ ratio between 1.4 and 1.2 failed due to lateral torsional buckling (LTB). While the beams with an S/D₀ ratio of 1.1 failed due to lateral distortional buckling (LDB). Furthermore, it was also observed that several openings with S/D₀ ratio of 1.3 have exhibited a considerable increase in failure load.