Through numerical simulation, it is demonstrated that, at zero averaged path dispersion managed optical fiber links, after both second order dispersion and third order dispersion are full compensated, nonlinear effect can make optical pulse narrower. So, optical pulse's peak power has to be kept very small. However, when polarization mode dispersion is considered, the nonlinear effect can be balanced. As a result, the system can be operated in higher power, so that signal to noise ratio of system will be improved.