Investigation of the performance of electronic equalization applied to receiver in the optical communication systems is introduced. Compared with other digital equalizers, maximum likelihood sequence estimator (MLSE) equalizer seems to be the most promising candidate solution to mitigate impairments caused by chromatic dispersion (CD), polarization mode dispersion (PMD) and fiber’s nonlinearity. The performance of a novel MLSE digital equalizer technology is analyzed and it is combined with feed-forward equalizer (FFE) receivers in optical 40 Gb/s polarization-maltiplexed (Pol-Mux) RZ-DQPSK systems. The MLSE receiver combined with FFE technique can achieve a higher CD tolerance and a long haul transmission with 280 km without any optical dispersion compensation and meanwhile the bit error ratio (BER) of the receiver is less than 10-3.