Abstract: The similarity analysis for the N S equations and the boundary conditions of a rotor stator system has been completed which shows that the flow field is depedent on five parameters such as inlet flow coefficient,rotational Renolds number,gap ratio,shroud clearance ratio and inlet diameter ratio.The numerical results for the laminar flow field in the system with an axial inflow and a radial outflow have been obtained by the SIMPLE method.It is shown that the gap ratio and the shroud have a great effect on the existence of the rotational core;and the core tends to take form and increase its rotational speed with decreasing the flow coefficient or with increasing the rotational Renolds number.For the closed rotor stator system the results are in good agreement with earlier work done by the other authors.Both the similarity analysis and numerical computation have testified the conclusion that the radial velocity is dominated by inertial terms and the tangential one is dominated by both inertial and viscous terms.The minimum of the flow coefficient required to prevent the ingress of gas into the rotor stator cavity has been calculated,and its linear relation with the rotational Renolds number has been found,which is consistent with the published experimental data very well.