Polarization sub-carrier modulation(SCM) is used to simultaneously monitor chromatic dispersion(CD) and polarization mode dispersion(PMD) in optical fiber communication systems.In transmitter,two pilot tones with different frequencies are added by using polarization-modulation method,and the powers of the two pilot tones are detected in receiver.It is verified that RF output powers increase with GVD and DGD,and the power radio of the two pilot tones increases with GVD and decreases with DGD.Thus,CD and PMD can be distinguished and monitored simultaneously.