Synchronously pumped mode-locking can be obtained when the frequency for gain switching of the pump-laser is equal to (or an integer multiple of) the cavity mode spacing, corresponding to the harmonic mode-locked status. A synchronously pumped mode-locked yetterbium-doped fiber laser (YDFL) is obtained when the pump semiconductor laser current is modulated sinusoidally at the appropriate frequency. By modulating frequency of the pump laser narrower output pulses are obtained on the second harmonic and rational harmonic mode-locked status and the 4th-order rational harmonic mode-locking status. At the repetition rate of 625 kHz,the pulse width of second harmonic mode-locked pulse train is less than 20 ns, approximately 1/40 of the pump-pulse-width. The average output power is about 2.34 mW and the energy convert efficiency is about 5%.