Abstract: The oscillating heat pipes (OHPs) are complex two-phase systems which have multiple parameters and multi-disciplinary physics，and investigations on OHPs are still at a primary stage.In this paper, a detailed theoretical model was developed by analyzing the forces and the heat and mass transfer in the systems. An n-shaped OHP with a single water plug and two vapor bubbles was investigated, and the governing equations were solved by using an explicit finite difference scheme to simulate the oscillating behavior of the liquid plug and heat and mass transfer characteristics.The results showed that the overall and local oscillations and the sudden increase and subsequent decrease in amplitude and frequency were the typical behavior of the liquid plug.The pressure difference between two bubbles was the main driving force of the oscillating plug.Capillary force could be neglected in comparison with other forces, and the plug gravity must be considered when OHP did not work horizontally.Instantaneous dry-up of the liquid film could enhance the amplitude and frequency of plug velocity and the overall heat transfer rate.