A novel scheme to generate inverse return-to-zero-differetial quadrature phase-shift-keying (IRZ-DQPSK) modulation format is introduced. The IRZ signal generated in this scheme does not contain any original phase information, so it is convenient to combine IRZ modulation with the phase modulation, and both the duty cycle and the extinction ratio of the IRZ-DQPSK signal are tunable. The influence of self phase modulation (SPM) is compared between IRZ-DQPSK and amplitude shift keying-differetial quadrature phase-shift-keying (ASK-DQPSK) in a 40Gb/s single channel long distance transmission by simulation. Results show that IRZ-DQPSK signal offers much better performance against SPM.