The discharge mechanism of the plasma electrolytic oxidation (PEO) process in different electrolytes was investigated by examining the variation of the optical emission spectra (OES). The spectrum of active species existed in the bubble layer. The bubble layer was initially broken down, followed by the breakdown of the dielectric barrier layer. Breakdown is the initial stage of discharge. A micro-discharge formation model, which assumes that the discharge ignition in the bubble layer developed at the oxide/electrolyte interface, was proposed. The active plasma species that appeared in different electrolytes during the PEO process were also studied. The appearance order of the excited active plasma species depended on the energy that the orbit transition of the species needed, but was not related to the anion concentration in the electrolyte. The anions in the electrolyte, except the OH -, also had little influence on the composition of the active plasma species during the PEO process. The active plasma species were mainly composed of metal atoms, metal cations, and gases produced by water decomposition. The electron temperature of the excited hydrogen was between 6 Γ 10 3 and 3 Γ 10 4 K. The high temperature provided the possibility of ceramic film melting and sintering. The source and transition of the active plasma species were also studied. They were found to undergo dissociation, ionization, and excitation processes.