Chirped return-to-zero differential phase-shift keying (CRZ-DPSK) modulation format under different channel spacings is investigated and compared with RZ-DPSK in 40×40-Gbit/s dense wavelength-division multiplexing transmission systems. An optimization operation over a wide range of parameters, such as span input power, precompensation percent, and chirp parameter (only for CRZ-DPSK), is performed in the simulation. Simulation results show that CRZ-DPSK demonstrates better performance in transmission distance, nonlinear, and dispersion tolerance with wider channel spacing of 100 GHz. When the channel spacing is reduced to 75 GHz, a slight degradation is observed for CRZ-DPSK and the performances of both formats are acceptable. But when it is reduced from 75 GHz to 50 GHz, a clear degradation of the CRZ-DPSK performance is measured, but RZ-DPSK demonstrates no significant degradation.