We demonstrated the QPSK signal generated by a single phase-modulator (PM-QPSK) is more tolerant to nonlinearity than that generated by the Mach Zehnder Interferometer (MZI-QPSK). With simulation of WDM system at 28 Gsymbol/s per polarization the performance of PM-QPSK and MZI-QPSK are compared when dispersion management is used. After 800 km's transmission in single-polarization transmission system, MZI-QPSK outperforms PM-QPSK because of its better tolerance to ISI. After 400 km's transmission in polarization multiplexing system, PM-QPSK outperforms MZI-QPSK by comparing the symbol error ratio with optimized launch power, which is because PM-QPSK has better nonlinearity tolerance in dispersion managed system.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.