In this paper, we report the gain-switching operation in Yb<sup>3+</sup>-doped DFB fiber laser. Our DFB fiber
laser operates with single longitudinal mode and single polarization state under cw pump, however, the
DFB fiber laser will operate with gain-switching mode if it is pump by pulse of certain duration.
Detail experimental results are given in the paper. We believe this king of pulse source is useful in