The assessment of turbulence models including a second-moment closure and the standard k-ε model has been performed for calculating a turbulent separated flow over a surface mounted obstacle. The results indicate more complex characteristics in this problem compared to backward facing step flow which is often used as the test case of turbulence models. Although the required computational effort is much higher than that of the k-ε model, the superior performance of the second-moment closure model is evident in representing the velocity field immediately above the obstacle as well as the pressure variation along the channel. It is indicated that the shortcomings of the k-ε model are attributable to the erroneous behaviour of the production rate of turbulent kinetic energy as calculated according to the eddy viscosity assumption.