Plasma structures, as well as vibrational and rotational temperatures of millimeter-wave discharge plasma in Microwave Rocket were investigated experimentally using a 28 GHz gyrotron beam. Four plasma structures were observed, including a comb-shaped structure in the range of 10–100 kPa and 0.01–1.0 GW/m2, which is a typical plasma structure observed in millimeter-wave discharge plasma. Besides, it was found that vibrational temperature didn’t exceed 6000 K at the beam intensity of 0.067–0.46 GW/m2 at atmospheric pressure, and non-equilibrium between vibrational and rotational temperatures was confirmed. Furthermore, a two-dimensional CFD was conducted, considering non-equilibrium temperatures and the plasma structures. The computational results suggested that over 2.0 GW/m2, an ionization-wave front can exceed a pressure-spike front in 1–10 mm order.