A new theoretical model for the convergent nozzle ejector in the anode recirculation line of the polymer electrolyte membrane (PEM) fuel cell system is established in this paper. A velocity function for analyzing the flow characteristics of the PEM ejector is proposed by employing a two-dimensional (2D) concave exponential curve. This treatment of velocity is an improvement compared to the conventional 1D "constant area mixing" or "constant pressure mixing" ejector theories. The computational, fluid dynamics (CFD) technique together with the data regression and parameter identification methods are applied in the determination of the velocity function's exponent. Based oil the model, the anode recirculation performances of a hybrid PEM system are studied under Various stack currents. Results show that the model is capable of evaluating the performance of ejector in both the critical mode and subcritical mode. (C) 2009 Elsevier B.V. All rights reserved.