A compact 200 W level diode-side-pumped microsecond (µs) pulse linearly polarized rod Nd:YAG laser oscillator was demonstrated with nearly diffraction-limited beam quality. The oscillator was based on a thermally near-unstable cavity design with two concave lenses in the cavity to enlarge the volume of the fundamental mode, leading to improvement of the laser efficiency and beam quality. Consequently, a record-high average power of 222 W was obtained at a repetition rate of 400 Hz with a 180 µs pulse width, corresponding to an optical-to-optical (o-o) conversion efficiency of 37%. The average beam quality factor was measured to be M2=1.32, resulting in a brightness value as high as of 11.25GW/sr⋅cm2. To the best of our knowledge, this represented the highest average power, the highest o-o efficiency, and the highest brightness for a µs pulse 1064 nm rod Nd:YAG laser oscillator.