The mechanism of perpendicular magnetic anisotropy (PMA) in a MgO-based magnetic tunnel junction (MTJ) has been studied in this article.By comparing the magnetic properties and elementary composition analysis for different CoFeB-based structures,such as Ta/CoFeB/MgO,Ta/CoFeB/Ta and Ru/CoFeB/MgO structures,it is found that a certain amount of Fe-oxide existing at the interface of CoFeB/MgO is helpful to enhance the PMA and the PMA is originated from the interface of CoFeB/MgO.In addition,Ta film plays an important role to enhance the PMA in Ta/CoFeB/MgO structure.