Abstract: The state-to-state approach, which includes the transitions between vibrational energy levels, state specific chemical reactions, and radiative transitions, is used to study the nonequilibrium process of O₂/O mixture system. The time evolution of the density of O and O₂ at specific energy levels as well as the radiative band intensities are numerically simulated for a stationary closed O₂/O mixture system with different initial conditions at constant governing temperature and volume. The transient properties such as the relaxation time and the equilibrium steady state values of the nonequilibrium processes are analyzed, and the interactions between the vibrational transitions, chemical reactions, and the radiative transitions are discussed. The results show that, the time evolution of the vibrational temperature does not follow the exponential approaching behavior described by the two-temperature model, the vibrational temperature even show non-monotonous variation with time, and the population distributions of the vibrational energy levels also depart from the Boltzmann distribution under the vibrational temperatures. The radiation properties show strong nonequilibrium feature in the transient process for the cases at high temperature, however, the effects of the radiation on the thermo-chemical processes are not evident, and the characteristic time of nonequilibrium radiation is one order of magnitude higher than the vibrational relaxation time.