A series of bcc-Co(001)/Ge/Co trilayers was grown on a GaAs(001) substrate by the molecular beam epitaxy technique. The optical and magneto-optical properties of the samples were studied. An oscillatory variation of the magneto-optical Kerr effect (MOKE) with the thickness of the Ge spacers was observed. We conclude that the effective optical constants as well as the real and imaginary off-diagonal element of the dielectric tensor should be considered in explanation of the MOKE activities since both have a large influence on the MOKE oscillations. Moreover, the coercivity and magnetization also oscillate with the Ge layer thickness.