Abstract: A theoretical model is established to calculate the dissociation degree of oxygen along the stagnant streamline of airflow, which is used to study the chemical nonequilibrium phenomena during hypersonic flight. The model assumes that nitrogen will not dissociate before oxygen undergoes serious dissociation. The recombination reaction in the boundary layer is also excluded in the model. It is found that, the dissociation degree of oxygen first increases and then decreases as the flight altitude increases, which is due to the competence between the equilibrium shift and nonequilibrium effect. This observation is confirmed by CFD simulations, and can be used to explain the phenomenon of the decline of real gas effects with increasing flight altitude in the literature. Using the developed model, the maximum dissociation degree of oxygen and temperature at the external edge of the boundary layer, are evaluated for hypersonic flows over a blunt body that cover a wide range of flight speeds and altitudes. The results are useful for engineering applications.