Abstract: Large-eddy simulation (LES) technique was employed to simulate flows around an isolated NACA 0025 airfoil at three different Reynolds numbers 55000, 100000 and 150000. The Mach number and angle of attack are 0.4 and 5 degree, respectively. Numerical results in three working conditions were compared with each other and analyzed to investigate the aerodynamic characteristics of the separated flows. Two different flow regimes exist around airfoil. A large open separated bubble appears on the upper surface of the airfoil when the Reynolds number is 55000 or 100000. As the Reynolds number increases to 150000, the time-averaged open separated region becomes a smaller closed separation bubble, and the wake width significantly decreased. For two flow patterns, the Kelvin-Helmholtz (K-H) instability plays an important role in the process of laminar separation induced transition. The increment of Reynolds number leads to accelerated transition and a closed time-mean separation zone altered from an open one.