Rib geometries are of interest in a variety of gas-liquid two-phase flow applications. Slug flow is one of the most prevalent but problematic two-phase flow pattern in industrial applications as it involves severe pressure fluctuation. In this study, the effects of rib geometry on pressure fluctuation and slug frequency, which are the main characteristics of the slug flow, were investigated, and the effects on controlling the slugging phenomenon is introduced. The characteristics of the slug flow vary greatly with time, so a careful statistical method was applied to grasp the effect of the lip structure. Regardless of the dimension of the rib geometry, the pressure fluctuation of the slug flow is generally weakened in the ribbed channel and the main frequency disappears. It was also confirmed by flow visualization that this phenomenon was caused by small air bubbles generated by the rib geometry. Depending on the width-to-height ratio of the rib geometry, the mechanism by which these phenomena occur was different, and the level of mitigation effect on slugging was different accordingly.