Thanks to the advantages of compact, high beam quality and high conversion efficiency, narrow linewidth high pulse energy single mode nanosecond-pulsed fiber amplifier is highly desired in laser detection and industry applications. Recently, an all-fiber amplifier with a home-made Yb-doped tapered fiber produces linearly polarized (12.3 dB PER) 100 ns, 1 mJ pulses with 0.49 nm linewidth, M²=1.49, 100 W of average output power and 10 kW peak power at 1 064 nm. It is believed that the pulse energy, output power and beam quality can be further improved by optimizing the Yb-doped fiber and design of amplifier.