A four-wave mixing effect on high-power continuous-wave fiber lasers has been demonstrated theoretically and experimentally. Detailed theoretical description of phase matching is presented and we found that the phase matching condition is satisfied at the frequency shift of 5.16 THz. While the intensity in fiber core region is more than about 394 MW/cm(2), the four-wave mixing products of 1100 nm and 1060 nm were also observed in high-power all-fiber laser. The comparison shows that the experiment result is in good agreement with the simulation result. In addition, the beam quality deterioration for the laser is caused by the four-wave mixing effect and the mode instability. The M-2 factor measured at maximal intensity of 478 MW/cm(2) is 2.80.