In this paper, two-phase air–water flow was investigated experimentally and simulated numerically using VOF method. The tests are conducted in Multiphase Flow Lab. of Tarbiat Modares University. In order to evaluate the rib effect on flow regimes, experimental investigation was conducted with ribs of different width and pitch where assembled on front and back side walls (side walls) of the duct during different test runs. The rib width and pitch were held constant during each test. The experimental work considered for different regimes of wavy, plug and slug which generated in the ducts with and without rib applying various phase velocities. The effects of using ribs on regime boundaries are presented in the flow diagrams and discussed in details. Compared to the smooth duct, the ribbed duct affects the different regime boundary positions noticeabily. The results showed that in the duct with small sizes ribs, the first slug initiates at longer time and distance in compare to the duct equipped with bigger size ribs. The results show that for normal operational flow velocities, the ribbed duct decreases the slug area on flow diagram map in compare to smooth duct. However, ribs facilitate the slug regime initiation for phase velocities in accordance with slug generation, which is not benefit of operational condition.