This article describes the strength behavior of C/SiC composites under non-stress oxidation. The oxidation kinetics model of a ceramic matrix composite contains both a reaction-controlled process and a diffusion-controlled process. The oxidation process has an effect on the fiber volume fraction, radius, and the composite interface shear stress. We obtained the residual strength of unidirectional C/SiC composites under 400-900 A degrees C oxidation with non-stress environments by combining the mass loss rate model with the strength failure model at room temperature. This model considers the different components of the system to be subject to oxidation at different temperature ranges. Then, the influence of oxidation temperature, oxidation time, and fiber volume fraction on residual strength was studied. Experiments on C/SiC composites at 650 and 800 A degrees C in an air environment with non-stress conditions were then performed, and the theoretical results of C/SiC composites were in good agreement with the experimental data.