In the present experimental study, the effect of surface wettability on pressure drop of two-phase plug flow in round mini-channels was investigated. With air-water mixture, glass, Teflon and polyurethane tubes were tested, where the tube diameters and the contact angles ranged from 1.59 ㎜ to 2 ㎜ and from 30° to 110°, respectively. In glass tube, liquid film was formed around the gas plugs. On the other hand, the gas plugs were directly in contact with the inner surface in Teflon tube. At low liquid flow rate, the structure of plug flow in polyurethane tube was the same in Teflon tube. The pressure drop of the plug flow in Teflon and polyurethane tubes appeared much larger than in glass tube. Also, in the plug flow regime with dry area, the pressure drop became higher as the contact angle was decreased. In this paper, the experimental results were compared with the previous correlations and the physical interpretations on them were provided.