The γ-Reθ transition model with a newly developed transition correlation set is applied to the stall angle prediction of thin airfoil, NACA 64A006. Empirical correlations used in the present model are developed based on literatures and numerical experiments and validated by skin friction coefficients on flat plate for several free stream turbulent intensities: T3 test cases for bypass transition regime and Schubauer& Klebanof for natural transition regime. For validation of separated flow transition, T3LC test case which uses coupland flat plate are computed and have a good agreement with experimental results of skin friction coefficient. The present γ-Reθ transition model is applied to stall angle prediction of NACA 64A006 airfoil to investigate the effect of boundary layer transition on massively separated flow problem within the framework of Reynolds-averaged Navier-Stokes computation. The results show that the γ-Reθ transition model improves the prediction capability of stall angle.