A model for predicting residual stiffness of unidirectional SiC/SiC composite in a stress-oxygen environment over 900 degrees C is developed based on an improved two-stage oxidation kinetics model. Two differential equations relating to the oxygen concentration are derived according to the conservation of mass. As a result, the thickness of the oxide on the fiber, the matrix, and the length of the interface consumed by oxidation can be calculated. The elastic modulus of the fiber and its stress redistribution are analyzed considering the change of the crack in different stress conditions, the oxidation defect on the fiber, the recession length of the interface, and the residual stress caused by the interface debonding. The residual stiffness of the SiC/SiC composite versus time after oxidation is drawn as curves under different stress levels which shows that the applied load does not affect the oxidation behavior of the matrix; it only changes the oxidation time of the composite. In addition, the residual stiffness of the composite decreases rapidly in the beginning for a while, then the degenerate rate stays stable until the interface is fully consumed. The predicted residual stiffness from the model is consistent with the experimental data.