In this paper, we propose and demonstrate optical signal to noise ratio (OSNR) monitoring method for polarization-division-multiplexing (PDM) signal by using the uncorrelated signal power, which is generated by balanced subtraction in electrical domain. The proposed OSNR monitoring is insensitive to dispersion impairment by using low bandwidth receiver. The proposed OSNR monitoring method is tested from 5 dB to 27.5 dB in 100-Gb/s PDM-QPSK system experimentally.