We have developed a high-power single mode monolithic tandem-pump fiber source operating at 1018 nm by employing a low core/cladding diameter ratio of 20/400 μm ytterbium-doped fiber. A record output power of 472 W is obtained, with a slope efficiency of 49.4% and near diffraction limited beam quality factor of M2 = 1.17. Careful numerical simulations were performed in order to determine the most suitable laser parameters such as reflectivity of output coupling fiber Bragg grating, length of active fiber and ytterbium ion concentration regarding a co-pumped oscillator setup. The signal output power behavior indicates that there are no limitations for further power scaling, except for insufficient pump power. This is apparently the highest recorded output signal power, efficiency and beam quality factor in monolithic 1018 nm fiber lasers using 20/400 μm ytterbiumdoped fibers. This structure can also be utilized as a high brightness pump fiber source of core-pumping ytterbium-doped highpower fiber amplifiers in a monolithic setup.