A stable ns pulse has been generated from a self mode locking Yb doped fiber laser using Yb doped fiber Bragg grating with 15 dB reflectivity as a reflector, and the pulse width is less than 5 ns. The average output power about 3 mW and repetition rate of 25 MHz are obtained under stable mode locking condition. The laser threshold is 18 mW and energy convert efficiency is 5%. The cause and characteristics of the self mode locking fiber laser have been also investigated.