In this paper, the Yb-doped fiber amplifier (YDFA) and its amplifying characters are reported. In order to study the characters of the YDFA, a narrow linewidth laser was used as the input signal, which produced by the single frequency DPL or narrow wavelength-width LD, operated in CW mode (DC signal) and pulse modulated mode (AC signal). The gain more than 26 dB with the fiber only eighteen meters was obtained.