The formulas for calculating nonlinear phase noise are proposed for both dispersion pre-compensation and post-compensation schemes. Based on these formulas, the phase noise, power tolerance and optimal signal peak power of both dispersion compensation schemes are analyzed and discussed in detail, respectively. Compared with the post-compensation scheme, pre-compensation scheme is more effective in reducing the nonlinear phase noise. Its suppression ability is improved with signal energy, amplified spontaneous emission (ASE) power spectral density and transmission distance increasing. The pre-compensation system possesses higher power tolerance than post-compensation system, and the optimal signal power is increased when dispersion is taken into account, which results in the optimal phase shift greater than 1 rad. And the optimal signal power for pre-compensation system is larger than post-compensation system.