A non-volatile memory element that is called a perpendicular-anisotropy magnetic tunnel junction has been fabricated in a CoFeB/MgO/CoFeB film stack technology. It exhibits two stable resistance values, high or low, depending on the relative direction of magnetization of the two ferromagnetic CoFeB layers. After programmed into the high resistance state with a current injection scheme based on the spin transfer torque theory, the tunnel junction has been exposed to 15-MeV Si ions in different voltage stress conditions. It has been observed that the tested structure remains in the programmed high resistance state after received the bombardments of several tens of Si ions and more, even under the stressed situations. A time-domain analysis has proven that this result is due to the perfect immunity of the tested magnetic tunnel junction to single event upsets. A resistance degradation due to the heavy-ion irradiation has been detected through a precise parameter analysis based on a tunneling theory but negligibly small, 1%.