© 2016 Elsevier Ltd. All rights reserved.Pulsating or otherwise known as oscillating heat pipes (OHPs) are two-phase heat transfer devices that can offer simple and reliable operation (no moving parts and vibration-free) with high effective results. This paper presents findings from an experimental flow visualization study into the various types of flow patterns within a closed-loop OHP with Fe2O3/kerosene as the working fluid. The complete transitions from nucleate boiling to formation of liquid slug and vapour plug were observed in the closed-loop OHP. From the flow visualization results, it was deduce that the motions of liquid slugs and vapour plugs are complex and influenced by many factors, such as heat rate, filling ratio and tube diameter. The findings also revealed that bubble generation and bubble growth induced a large driving force which contributed to the random motion of liquid slugs and vapour plugs. The flow visualization also revealed that bubbles with diameter equal to the inner diameter of the tube, otherwise known as tube-size or TS bubbles, were generated when the velocity of liquid slug was lower or equal to 0.15 m/s and the velocity of the liquid-vapour interface increased as TS bubbles were generated.