A first MRAM set includes a first transistor and a second transistor. The first transistor includes a first gate structure, a first source/drain doping region and a first common source/drain doping region. The second transistor includes a second gate structure, a second source/drain doping region and the first common source/drain doping region. A second MTJ is disposed on the second transistor. The first common source/drain doping region electrically connects to the second MTJ. A first MTJ is disposed on the first transistor. The sizes of the first MTJ and the second MTJ are different. The second MTJ connects to the first MTJ in series. A bit line electrically connects the first MTJ. A source line electrically connects to the first source/drain doping region and the second source/drain doping region.