Abstract: The tough-brittle transition is important for the design and performance evaluation of continuous fiber-reinforced ceramic matrix composites (CMCs). Using a single-fiber cylinder model together with elastic mechanics theory, shear-lag theory, fiber statistical strength theory and the rule of mixture, a trans-scale characterization and prediction model for the tough-brittle transition tensile strength of CMCs is established, taking into account the microscopic damage and failure mechanisms, fiber near-surface stress concentration factor and interfacial debonding energy release rate. The theoretical model is then used in 2D-C/SiC composites for preliminary comparative demonstration. The calculation results show that the mixing-rule strength model can reasonably characterize the tough-brittle transition strength properties of CMCs and accurately predict the interfacial mechanical conditions of the tough-brittle transition of CMCs. The theoretical strength prediction values for 2D-C/SiC composites are in good agreement with the experimental data, which reveals the core mechanism of the tough to brittle transition of CMCs is the fiber near-surface stress concentration effect. In addition, when the influence of fractured fiber clusters on stress concentration is considered, the strength predictions of the model decrease substantially, indicating that the non-uniform interfacial properties and the localized fracture nuclei of fibers breaks due to non-uniform load bearing have a significant weakening effect on the load-bearing properties of CMCs.