In order to study the heat transfer mechanism of miniature oscillating heat pipes (MOHPs) and predict the heat transport capability of MOHPs, a comprehensive mathematical and physical model of MOHP was built to simulate the two-phase flow behavior in vertical bottom heating mode. Water was used as the working fluid. The volume of fluid (VOF) and mixture model in FLUENT were used for comparison in the simulations. The phase change process in a MOHP was deal with by adding a user-defined function (UDF) source term in each phase. The continuum surface force (CSF) model was used to consider the effect of surface tension. The result showed that the mixture model was more suitable for the two-phase flow simulation in a MOHP. Having agreed with the flow visualization, the simulation with unsteady model was successful in reproducing the two-phase flow process in a MOHP, including the bubble generation in evaporation section and the oscillations caused by the pressure difference. The quasi periodic thermal oscillation with the same characteristic frequency for both evaporation section and condensation section indicated that the heat was transferred by the oscillations. The simulation results of MOHPs with different heat transfer lengths (L) and inner diameters (D-i) at different heating powers, were compared with the experimental results at the same condition. This showed that the inner diameter had a greater impact on the thermal performance of MOHPs than the heat transfer length. Increasing the inner diameter was beneficial to improve the thermal performance of MOHPs. (C) 2012 Elsevier Ltd. All rights reserved.