A three-dimensional unsteady thermal and hydrodynamic model is developed to numerically analyze the heat and fluid flow in flat-plate oscillating heat pipe (FP-OHP) which is a promising effective heat transfer component for building energy conservation. The vapor-liquid two-phase flow patterns and temperature distributions in the FP-OHP with various filling ratios are presented, and especially the detailed bubble behaviors and size distribution are investigated. It is indicated that the dispersed bubbles in FP-OHP are generally produced by the nucleate boiling in evaporator and the condensation of short vapor plugs in condenser. The short plugs are usually formed by the self-growth and coalescence of dispersed bubbles. Additionally, the long plugs occur due to further coalescence of the short plugs. The proportion of dispersed bubbles decreases and then increases with increasing heat load, and the average size of total bubbles is inversely proportional to the filling ratio. The optimal filling ratio for the thermal performance of FP-OHP is determined by most adequately combining the advantages of the sufficient bubbles pumping action for driving the heat transport along with the motion of working fluid, and the sensible heat transfer of the liquid, which is shown around 50%. The reasonability of the present model is experimentally verified. (C) 2014 Elsevier B.V. All rights reserved.