A novel scheme for wavelength monitoring of DWDM systems was proposed in which the two transmission spectra of a polarization interference filter (PIF) are employed. The filter consists of a polarizer, a polarizing beamsplitter (PBS) and a birefringent plate sandwiched between them. The birefringent plate used is made of yttrium orthovanadate (YVO4) crystal. The two transmission spectra with perpendicular polarization directions are complementary in light transmittivity. By designing the filter and controlling its temperature appropriately, the intersections of the two transmission spectra can be utilized to monitor operating wavelengths in a DWDM network. This scheme doesn't need reference light for wavelength sensing. In addition, the monitoring sensitivity is twice as large as that in the way that only one transmission spectrum is utilized.