The oxygen partial pressure during the sputter process of TbFeCo thin films was controlled precisely. The effect of the oxygen partial pressure on the perpendicular magnetization of TbFeCo films was quantitatively studied. Between 5.4 X 10(-6) mbar and 1.29 x 10(-5) mbar oxygen partial pressure, the films lost perpendicular magnetization rapidly and became paramagnetic. Below this range, oxygen had no effect on the perpendicular magnetization of the films.