Self-pulsation behavior of Yb3+-doped double clad fiber lasers formed in Fabry-Perot cavities and end-pumped by CW laser diode (LD) is investigated. Two kinds of self-pulsation are observed in the fiber laser with a low Q cavity, one is the pulse generated from passive Q-switching of saturable absorption;the other is the giant pulse due to stimulated Raman scattering (SRS) in the fiber. It is demonstrated that by promoting the Q value of the cavity the pulsation in the output of the fiber laser can be reduced and stable CW output will be obtained while an appropriate rate of pump is applied simultaneously.