A design of passively mode-locked fiber laser which can produce nanosecond pulses with high repetition frequency is presented. Inserting a semiconductor saturable absorber mirror (SESAM) in a Fabry-Perot (F-P) cavity, using all negative dispersion fiber, without any pulse compression mechanism, a pulse whose width is 1 ns and repetition frequency is 10.74 MHz is achieved at a pump power of 75 mW. The spectral analysis shows that the 3 dB width is smaller than 0.1 nm. When the pump power goes to 130 mW, a maximal power of about 20.5 mW is generated. The experiment indicates that the passively mode-locked fiber laser with an SESAM can obtain a nanosecond pulse with high repetition frequency when all negative dispersion fiber is used.