Abstract: A new low Reynolds number nonlinear K-ε model is presented,in which two correction functions for the near-wall flow and a nonlinear expression for the Reynolds stress are incorporated.Several backstep separated flows with two different expansion ratios are numerically studied by using the above model and a standard K-ε model.In comparision with experimental data,the results obatined show that the predictions of the presented nonlinear model are obviously improved over these of the standard K-ε model in respect of various flow parameters,such as reattachment length,time mean velocities and Reynolds stress components.Also,the presented model is the partial differential Reynolds stress model in prediction of the Reynolds stress components.