Abstract: The in-plane shear mechanical behaviors and mesoscopic damage behaviors of plain woven SiC/SiC and C/SiC composites were investigated with Iosipescu specimens.The materials' monotonic and hysteresis stress-strain responses in the various directions were obtained by tests, and the shear damage properties of these two materials were analyzed comparatively.It is found that the shear damage evolutions of materials are affected by residual thermal stress significantly.SEM(scanning electron microscope) photos of fracture surfaces indicate that the bridging fibers withstand significant bending load and deflection.Therefore a fiber bending-bearing mechanism was proposed.And combined with the crack closure effect, the shape of hysteresis loops was elucidated well.By considering the characteristic of mesoscopic shear damage modes, two damage variables were defined to character the shear damage evolution of material.And the shear damage evolution models of these two materials were developed.The results show that the initiation stress for matrix cracks in the 45°direction of 2D-C/SiC composite is much lower than that of 2D-SiC/SiC composite, but the initiation stress values for matrix cracks in the 0°/90°direction are the same for these two materials.