To investigate the distortional buckling performance of castellated beams, six castellated beam specimens were subjected to static load at the mid-span. The corresponding failure modes, load-vertical displacement responses, and web deformed shape curves of specimens were obtained. The results indicate that all six specimens failed in a distortional buckling mode. Specifically, the compressive flange of the entire specimens exhibited an S-shaped curve upon failure. Meanwhile, out-of-plane bulging deformation occurred in the web post section at the mid-span loading location. With the increase of castellated beam length, both the initial stiffness of the load-vertical displacement curve and the critical distortional buckling capacity reduced. Results of the distortional buckling strengths obtained from the experiments were further compared with the predications calculated using the modified method suggested by AS4100, AISC, and Nethercot and Trahair. The comparison results show that the predications of the AS4100 method and the Nethercot and Trahair approach are in good agreement with the test results.