Phase pre-modulation is investigated to overcome the impacts of sensitivity reduction and chromatic dispersion in high-speed and long-distance fiber transmission systems. The bit-synchronous phase pre-modulation imposed on non-return-to-zero (NRZ) signals at the transmitter can reshape the NRZ pulse to return-to-zero (RZ) waveform and hence improve the eye opening. Experiments are performed to measure the spectra, eye diagrams and power penalties of conventional NRZ signals and phase-pre-modulated NRZ signals under different modulation depth and dispersion. Results of 10 Gb/s transmission show that the phase-pre-modulated NRZ signals have a power penalty lower than 1 dB with the absolute value of link dispersion less than 1000 ps/nm, exhibiting higher sensitivity and lower dispersion-induced penalty than conventional NRZ signals. This modulation scheme relaxes the requirements to optical signal-to-noise ratio and dispersion management, extends the transmission distance and is easy to implement.