Plasma structures, as well as vibrational and rotational temperatures of millimeter-wave discharge plasma were investigated experimentally using a 28 GHz gyrotron beam. Four plasma structures were observed in the range of 10–100 kPa and 0.01–1.0 GW/m2 . Besides, the vibrational temperature didn’t exceed 6000 K at beam intensities of 0.067–0.46 GW/m2 and atmospheric pressure, and non-equilibrium between vibrational and translational–rotational temperatures was confirmed. Furthermore, a two-dimensional and two-temperature CFD simulation was carried out which took into account the non-equilibrium temperatures and plasma structures. The computational results suggested that an ionization-wave front can exceed a shock wave front on the order of one millimeter due to the energy relaxation delay from vibrational to translational temperature.