[en] The magneto-optical polar Kerr spectra of CoFeB/MgO thin films on different kinds of underlayers (Hf, Pt) were investigated in order to understand the origin of the interfacial perpendicular magnetic anisotropy (PMA) in that structure. The Kerr spectra were measured by using real-time Kerr spectroscopy based on a fiber-coupled high-speed spectrometer in the photon energy range of 1.5 ∼ 2.9 eV. The magneto-optical studies revealed that the magneto-optical proximity effect due to the Hf underlayer to CoFeB was weak, but the interfacial contribution of the Pt underlayer to the CoFeB was strong. The variation of the PMA as a function of the annealing conditions was also investigated, and a gradual enhancement of the PMA was observed with increasing annealing temperature for the Pt underlayer sample, but a gradual decay of the PMA was observed for the Hf underlayer sample. The magneto-optical studies showed that the formation of CoPt alloys at the Pt/CoFeB interface played an important role in the PMA of the Pt/CoFeB/MgO structure at higher annealing temperature.