Abstract: Combining with γ-Re_θ transition model, scale adaptive simulation (SAS) is successfully applied on the laminar separation flow past a circular cylinder at high sub-critical Reynolds number (Re=1.4×10⁵). Numerical results are statistically closed to experimental data, and especially the agreement of pressure recovery in separation zone is fairly well. The major error comes from the inaccurate prediction of the separation position. Transient flows indicate that there are spanwise instabilities in the shear layer of laminar separation flow, and they will grow to be fully turbulence when transporting downstream. Comparing to laminar separation, because of the lack of the unsteadiness related to the shear layer instability, the scale resolving capability of SST-SAS is weaker for turbulent separation flow. Therefore, the scale of turbulent structures resolved by SAS in the wake of turbulent separation is much larger than the laminar separation.