Subcooled boiling heat transfer, which is related to Departure from Nucleate Boiling (DNB), is highly concerned in fuel assembly of Pressurized Water Reactor (PWR). It is of importance and significance to predict void fraction and mass/energy transfer characteristics among sub-channels under subcooled boiling with 3-D CFD code to obtain more details for better understanding of two-phase flow process in rod bundle. To better understand and predict void distribution and its transportation characteristics in rod bundle, two-phase flow simulation with two-fluid model was carried out with Heat Partitioning Model. In this study, the 5×5 full length rod bundle of NUPEC PWR Sub-channel and Bundle Test (PSBT) with Uniform-Axial Power Distribution (UAPD) was used under prototype condition (test serial of B5 configuration) to test the physical method. The parameter distribution in axial and radial orientation among different sub-channels at the downstream of the last Mixing Vane Grid (MVG), where the DNB occurs at the end of the heated section, was studied. Different void fraction distribution was observed among central channels, side channels and corner channels. It is induced by different mass transfer process results from the layout of mixing vanes. Moreover, the intensity and direction of mass transfer process shows as a dominant effect on the void fraction distribution and development among subchannels. Energy transfer is related to the mass transfer among sub-channels. The development of two phase flow at the downstream of MVG and SSG indicate that the simple support grid (SSG) draws into a suppression of radial transverse flow and of mass transfer process, which are introduced by the mixing vanes for heat transfer enhancement.