Abstract The paper studies on the basis of VOITH R133-2 hydraulic retarder, the inlet and outlet structures of the oil passage on the stator are rearranged, which are made a more uniform structure distribution. In order to find out the characteristics of this kind of structure arrangement. The flow passage models for two different structures are established, and the internal flow field characteristics are studied by using the CFD (Computational Fluid Dynamics) method. The flow rules of the internal oil, the distribution of pressure field and velocity field as well as output braking torque are obtained. The results show that rearranged structure retarder has a more uniform pressure distribution and a lower output braking torque than original structure retarder. And the simulation verifies the effectiveness of simulating true flow by CFD in hydraulic retarder flow field and conduct retarder design and structure optimization. Introduction As a kind of the auxiliary brake, the hydraulic retarder is widely used in commercial vehicles for its merits of high braking torque and long time continuous working ability. A hydraulic retarder is very similar to a fluid coupling, it can be seen as special kind of fluid coupling which works at a zero slip ratio condition. he rotor brings the oil lash against the blades of the stator, and the braking torque is generated. During this time, the mechanical energy of rotation is converted into heat energy, and finally the heat is taken away by a radiator. It is very effective for a long distance braking [ 1]. As the flow in the hydraulic retarder is three dimensional, unsteady, and turbulent. It is very costly to measure the internal flow with experiment. With the development of Computational Fluid Dynamics (CFD) technology and computer hardware performance these years, many researchers start to study the internal flow field characteristics of retarder by CFD technology. Tao Yang, et al. [2] combined CFD and FEA (Finite Element Analysis) technology, analyzed strength, deformation, Von Mises stress and modal structure features of a hydraulic retarder blades, Xuesong Li, et al. [ 3] studied the volume distribution of the two-phase flow under different filling rate in the transit state. As some structure and working principle similarities with fluid couplings, when studying the hydraulic retarder, we can also refer to some fluid coupling and torque converter works. L. Bai et al. [4] numerically simulated the turbulent flows in fluid coupling, analyzed the vortex generation and its effect on the torque transmission, and the local velocity and torque were also computed. Y. Dong et al. [5] measured the pump and turbine exit flow field of a torque converter, they studied the free-stream flow, blade wake flow, core-suction corner separation flow and mixing flow four zones of the exit flow. R. Flack and K. Brun [ 6] used a rotating 2d straight-walled duct to model the pressure to suction side jet/wake flow due to rotational Coriolis force, and a 180°flow bend to model the core to shell side jet/wake flow due to rapid radial/axial flow turning for a torque converter pump. Table 1 shows some structure parameters of VOITH R133-2 hydraulic retarder. Fig.1 (a) shows the CAD model of original stator of the retarder. From Fig.1(a) we can find out that six oil outlets and ten oil inlets of oil distribution on the stator are irregular. Theoretically, this kind of structure is not very good for dynamic balance when rotating. So we rearrange the stator inlets and outlets, make them a more uniform distribution ( Fig.1 (b)). Here, we define two retarders, retarder A refers to the original retarder, and retarder B refers to the rearranged retarder. The rotors of two retarders are the same, that is, the only difference of two retarders is the structure of two stators. The paper studies internal flow field characteristics of this two kind structure retarders, and compares torq