In this paper, the effect of pipe rotation on two-phase flow patterns and pressure drop of a horizontal pipe is experimentally investigated. To obtain two-phase flow patterns and related transition boundaries, a unique experimental set-up was constructed to measure flow patterns under different pipe rotational speeds. A Plexiglas pipe, with 4000 mm length and 25.4 mm ID was used in experiments to allow direct observation of flow pattern. For rotation of the pipe, an electromotor coupled with gearbox was used to allow different rotational speeds. The set-up also supports up to +/- 25 degrees inclination. Air and water were used as the gas and liquid phase, respectively. Over 3800 experiments were conducted to draw flow pattern maps at six different rotational speeds of 0, 50, 100, 200, 300 and 400 rpm in both horizontal and 10 degrees inclined pipe. To validate the results, experimental findings were compared with previous research for a horizontal fixed pipe case. Results show that pipe rotation has a significant effect on the flow pattern map and the transition boundaries. It was found that in horizontal pipe case, the stratified smooth flow regime decreases as the pipe rotation increases and disappears at high revolution speeds. Moreover, the annular regime enlarges with increasing pipe rotational speed. For a 10 degrees inclined pipe, the stratified wavy region appears as the pipe rotation speeds up. It was observed that for both horizontal and inclined pipe, pressure drop considerably increases as the pipe rotational speed increases. Furthermore, the effect of pipe inclination on the pressure drop is reduced as the rotational speed increases. (C) 2018 Elsevier Ltd. All rights reserved.