An apparatus and a method for monitoring optical signal-to-noise ratio are provided. It can be applied in dense wavelength-division multiplexed networks to monitor the transmission quality of each optical channel. The apparatus comprises an optical circulator, a tunable optical filter, a dithering signal, a dithered reflector, and two photodiodes. It can be integrated on a single chip. The invention utilizes the dithering and reflection functions of the dithered reflector, and passes the signal and the noise through the tunable optical filter once and twice, respectively. When the tunable optical filter is scanning and filtering the whole spectrum, the signal and noise powers are measured by the two photodiodes, respectively. The OSNR for each optical channel is then calculated according to the signal and noise powers. It can monitor channel location, wavelength drift, and OSNR by including a wavelength locker to act as an optical channel analyzer.