High power single-frequency (SF) lasers are of great significance in science, industry, and defense applications such as gravitational wave detection, free apace optical communication, range finding, lidar, parametric wavelength conversion in resonant cavities. We report on a high power single-frequency linearly polarized radiation with a nearly diffraction-limited beam quality by use of a two-stage fiber master-oscillator power amplifier system. The master oscillator is an SF distribution feedback fiber laser with polarization maintaining (PM) and single mode (SM) fiber pigtailed output power of about 10 mW at 1064 nm, and its line-width is of 20 kHz. This light power is amplified to about 1 W by the first stage of all-fiber SM/PM amplifier and then seeded at the second stage of high power main amplifier with large mode area (LMA) ytterbium-doped fiber (YDF). The pumping diode laser of the main amplifier is fiber-coupled (the core diameter is 400 μm and corresponding numerical aperture is 022) with a max emitting power as high as 190 W at 976 nm. By employing the co-pump configuration with a space coupling efficiency of about 85%, we demonstrate that the power output from the LMA/PM YDF (core diameter is 20 μm, inner cladding diameter is 400 μm) reaches 128 W and its corresponding overall slope-efficiency is 85% of the launched pump power. The beam profile is diffraction-limited and linearly polarized with a polarization extinction ratio over 12 dB. There is observed neither amplified spontaneous emission nor stimulated Brillouin scattering phenomenon, which means that the substitution of the pump source with the more powerful one can further enhance power output if LMA YDF with a larger core size, shorter fiber length and higher doping efficiency is used.