Abstract: Two-dimensional transonic flow in a diffuser is numerically invesgated by solving 2D time-dependent Reynolds-averaged compressible Navier-Stockes equations.The numerical method is based on Van-Leer's flux splitting cell-centered finite volume scheme.Baldwin-Lomax algebraic turbulence model and Johnson-King non-equilibrium turbulence model are tested to simulate largely separated flow.Comparison of the computational results with corresponding experimental data shows that the predicted wall static pressure distributions and shock strength by both two turbulence models agree well with the experimental data,but Johnson-King model does better than Baldwin-Lomax model in the prediction of separation reattachment point.