A high power all-fiber laser is experimentally studied. A monolithic fiber laser system with only one stage of resonator cavity is constructed and 404-W continuous wave (CW) output power is obtained. Its opticalto-optical conversion efficiency is up to 64%. The laser central wavelength is at 1081 nm with the spectral full-width at half-maximum (FWHM) of 2 nm. The laser setup can work with excellent stability; in the long time of high power operation, no thermal distortions or damages are observed.