The two-dimensional electron system in strongly coupled GaAs/Al0.3Ga0.7As double quantum wells has been studied by Shubnikov-de Haas¡]SdH¡^measurements. The degenerate subbands of the double quantum wells are lift into two subbands with a symmetric or anti- symmetric z-direction wave function. We observed that the SdH oscillation due to the second subband of anti-symmetric wave function. The effective mass of the second subband is linearly dependent on the magnetic field range from 0.680 T to 1.964 T. The mass enhancement for the double-quantum-well with an equal well thickness is greater than that with an unequal one. This mass enhancement is attributed to the electron-electron interaction.