By using the birefringent crystal YVO_4, a novel scheme of wavelength monitoring in DWDM optical communication systems is presented. The principle of this scheme is analyzed and its feasibility is inspected with experiments. Theoretical analysis and experimental results both show that it can be applied in C-band wavelength monitoring and locking in DWDM systems. As compared with the method of wavelength monitoring by means of the transmission spectrum of a Fabry-Perot etalon, this scheme has advantages such as easy judgment of normal operation wavelengths, high sensitivity of monitoring and easy integration.