We investigate the magnetic anisotropy in as-deposited and annealed Ta/ CoFeB/ MgO samples prepared by sputtering and its CoFeB thickness dependence. The magnetic easy axis changes from in-plane to perpendicular with decreasing CoFeB thickness. The thickness, at which magnetic easy axis direction changes, is increased by annealing. It is also shown that the magnetic anisotropy can be modulated by electric field and its modulation ratio is larger for the annealed samples.