It is common to use Yb-doped double-clad fibers as gain material for high power fiber lasers. It is found that these fibers are similar to the general un-doped telecommunication fiber. Because of small core size, only from several microns to tens of microns, it is much easier to cause self-pulsing effects in the fiber lasers. It is presented the experimental study of self-pulsing behavior of Yb-doped fiber lasers. The result shows that under the high power condition without concentration effect, large core Yb-doped double-clad fiber are the same to three-level ion systems that self-pulsing behavior is the result of relaxed oscillation. To Yb-doped fiber laser, saturated absorption effect, stimulated Brillouin scattering and stimulated Raman scattering cannot be ignored.