We demonstrated a high-peak-power, single-frequency, single-mode, linearly polarized, nanosecond all-fiber laser in master oscillator power amplifier configuration. The pulsed seed was generated by modulating a single-frequency continuous wave laser at 1064 nm. The short pulse width (∼3.2 ns) of the pulsed seed prevented the onset of stimulated Brillouin scattering in the 10 μm-core single-transverse-mode polarization-maintaining ytterbium-doped fiber of the main amplifier. In order to compensate self-phase modulation-induced spectral broadening, a phase modulator was employed to control the phase of the pulsed seed, and the linewidth of the output pulses were reduced from ∼1.22 GHz to ∼185 MHz at the peak power of 1.47 kW.