In this study, the influence of inner surface wettability on thin liquid film behaviors of slug flow in small circular tubes, including flow pattern, liquid film thickness, pressure drop, were investigated. Visual observation of flow regime by using a high-speed camera and direct measurement of liquid film thickness of slug flow by using a laser confocal displacement meter were conducted. The inner surface of circular glass tube (inner diameter D = 0.5, 1.0, 2.0 mm) is chemically treated to be either hydrophilic or hydrophobic. In order to investigate the effect of the surface tension on the thin liquid film behavior, water and FC72 were selected as liquid phased while and air was used as gas phase. Experimental results for each experimental condition were compared and discussed to consider the influence of wettability, inner diameter, and liquid physical property to provide better understanding of two phase flow in small channel.