The subband structure and optical transition of GaAs/AlGaAs superlattices are investigated in the presence of an electric field applied along the growth direction and a magnetic field parallel to the GaAs/AlGaAs interface by using the effective mass theory. In the presence of the parallel magnetic field, the twofold degeneracy is removed and heavy and light holes are mixed. The presence of the electric field introduces the Stark shift. The magnetic-optical transition probability is also discussed.