The calcium aluminosilicate (CAS: anorthite) glass-ceramic matrix materials in the SiCf/CAS composites deformed in compression at a high temperature were analyzed for microstructures. The matrix region right above the top of the SiC fiber (θ ∼ 0°) showed high density of dislocations as well as twins. On the other hand, the matrix region remote from the top of the SiC fiber (θ ∼ 90°) showed only a twinned structure. This experimental result was compared with the Meyer et al.'s mechanical modeling indicating that the interfacial traction of the matrix region right on the top of the fiber is significantly high compared to the remote stress and thus it rate limits the creep deformation of the composite.