Closed loop two-phase thermosyphon is an effective heat transfer device, of which the research is necessary to clarify the heat transfer and fluid flow mechanism. Numerical analysis was carried out in the article with one-dimensional model. The model was based on mass, momentum and energy conversation equations, solved by iterative method. Single-phase and two-phase flow were simulated, besides supercritical flow. Experimental data was used in the validation of the model with the error less than +/- 10%. The axial evolution of several parameters, namely pressure, enthalpy, void fraction, vapor quality and heat transfer coefficient in the loop, were presented firstly. Then the effects of pipe diameter, filling ratio, height difference, heat load, cold bath temperature and fluid type on the closed loop two-phase thermosyphon were investigated. Large pipe diameter and medium filling ratio led to low thermal resistance. The thermal resistance decreased with the increase of height difference firstly, and then increased at large height differences. Larger heat load led to higher mass flux, as well as larger subcooled or superheated region. Vapor quality at the outlet of evaporator increased with the increase in cold bath temperature, leading to low thermal resistance. Fluids with low viscosity and large density difference between liquid and vapor phase were suggested for favorable heat transfer performance. It was also concluded that thermal resistance increased sharply when supercritical phenomenon occurred due to the disappearance of two-phase region. (C) 2019 Published by Elsevier Ltd.