Unidirectional SiC/SiC composites fabricated with highly-crystalline and stoichiometric fibers and matrix, but with three different interfacial types (single-layer pyrolytic carbon (PyC), multi-layered SiC/PyC, and pseudo porous SiC interfaces) were irradiated up to 1.0 × 1025 n/m 2 (E > 0.1 MeV) at 1073 and 1273 K. Tensile, inter-laminar shear, and flexural properties were evaluated to compare the role of different interfaces on neutron irradiation behavior. There was nearly no significant degradation in tensile and flexural strength after high-temperature neutron irradiation, except for porous SiC interphase composite. Moreover, no meaningful reduction of tensile modulus was identified regardless of interphase types, although 20-40 % degradation in flexural moduli occurred due to a reduction in inter-laminar shear modulus. In contrast, matrix cracking stress was significantly dependent on interfacial properties. Multilayer interphase composites exhibited the best irradiation stability. Irradiation instability of thick PyC and porous SiC interphase resulted in 20 % and 40 % degradations of matrix cracking stress, respectively. Copyright O 2005 by ASTM International.