A MoO3 saturable absorber based on Yb-doped mode-locked fiber laser which can operate in both continuous wave and Q-switched mode-locked regimes was reported. A stable continuous wave mode-locked pulse train at a repetition rate of 17 MHz was achieved by using a ring cavity configuration at 95 mW incident pump power, the pulse width, central wavelength and 3 dB spectral bandwidth are 130 ps, 1 067.06 nm and 0.27 nm. Stable Q-switched mode-locked pulse train occurred at 280 mW incident pump power. As the pump power varied from 280 mW to 400 mW, the repetition rate of Q-switched envelope varied from 26.51 to 48.7 kHz, and the width of Q-switched envelope varied from 14.6 μs to 4.1 μs, the pulse width and the central wavelength remained invariant. The 3 dB spectral bandwidth was 0.62 nm.