The inter-face tracking method and the particle interaction method can simulate detailed two phase flow in the rod bundle without any physical model based on experiment. However, evaluation of liquid films and small scale bubbles requires high resolution. Therefore, it is difficult to apply such methods to two phase flow analysis in the rod bundle, because of huge computational cost. [figure] [figure] Fig.1 Core design of the FLWR On the other hand, the two fluid model can simulate two phase flow less computational cost than the inter-face tracking method and the particle interaction method. Therefore, the two fluid model is useful for thermal hydraulic analysis in the rod bundle Japan Atomic Energy Agency (JAEA) develops three dimensional two fluid model analysis code ACE-3D, which adopts boundary fitted coordinate system in order to simulate complex shape channel flow, and which can divide the subchannel in the rod bundle into more than one block, and calculate with interaction between blocks. In order to simulate the two phase flow in the large scale domain such as the rod bundle, large number of meshes are necessary. Therefore, parallelization based on Message Passing Interface (MPI) is introduced to ACE-3D for the purpose of calculation time reduction. In order to investigate the parallelization efficiency, the computation with about 9 million meshes has been performed. It is confirmed that parallelization efficiency over 80% can be obtained with 252 CPU. In order to confirm that ACE-3D can be applied to the thermal hydraulic analysis in the rod bundle, the two phase flow analysis in the channel which simulates the rod bundles is performed. The obtained results are agreement with experimental result qualitatively. Therefore, it is concluded that ACE-3D can be applied to the thermal hydraulic analysis in the rod bundle.