The present paper deals with a study on the mechanism of capsizing phenomena of a semi-submersible platform in regular beam waves. Especially, the author's attension is paid to analysis of larger steady tilt of the platform under the excitation caused by the wave-induced vertical steady force on the lower hulls. Firstly, the steady wave force and moment on the submerged cylinder were investigated. As the result of this study, the steady heeling moment due to vertical steady force on the lower hulls were obtained by both theory and experiment. Secondly, the time histories of the platform motions predicted by the computer simulation are shown and are compared with the experimental results with good agreement. Finally, the insights of stability of a semi-submersible platform in regular waves are examined by means of graphical illustrations. In conclusion, it is clarified that the larger angle of steady tilt is strongly influenced by the wave-induced vertical steady force on the lower hulls. Especially, when the platform has small metacentric height and/or the higher position of fairleader, the angle of tilt increases drastically.