Slug flow has drawn attention for industrial applications because it is one of the most prevalent and problematic two-phase flows due to severe pressure fluctuations. The effects of rib elements installed at the inner walls of a channel on the flow structure and pressure fluctuations are experimentally investigated with particular focus on how the rib elements alter slugging phenomena as compared with a smooth channel without ribs. Statistical analyses are conducted to elucidate the relation between pressure fluctuations and flow dynamics of slug flow in channels with ribs. For the slug flow in the ribbed channels, pressure fluctuations are alleviated, and the dominant frequency of the peak power magnitude for the pressure disappears. Two distinct mechanisms for air bubble entrainment are suggested depending on the width-to-height ratio of the rib geometry. This accounts for different levels of pressure fluctuation mitigation and the increase in flow randomness as well as pressure loss along the channel.