Thermophysical and mechanical properties of high purity chemically vapor-deposited (CVD) SiC and chemically vapor-infiltrated SiC matrix, pyrocarbon/SiC multilayered interphase composites with Hi-Nicalon™ Type-S and Tyranno™-SA3 SiC fibers were evaluated following neutron irradiation. Specimens including statistically significant population of tensile bars were irradiated up to 5.3 displacement-per-atom at ∼220 to ∼1080 °C in the Advanced Test Reactor at Idaho National Laboratory and High Flux Isotope Reactor at Oak Ridge National Laboratory. Thermal diffusivity/conductivity of all materials decreased during irradiation. The reciprocal thermal diffusivity linearly increased with temperature from ambient to the irradiation temperature. The magnitude of defect thermal resistance was distinctively different among materials and its ranking was Hi-Nicalon™ Type-S > Tyranno™-SA3 > CVD SiC regardless of irradiation condition. Dynamic Young's modulus decrease for the irradiated CVD SiC exhibited explicit correlation with swelling. No significant effects of neutron irradiation on tensile properties of the composites were revealed, except for an anomaly case for the Hi-Nicalon™ Type-S composite irradiated in a specific condition. According to the single filament tensile evaluation, fibers of both types retained the original strength during irradiation at intermediate temperatures but significantly deteriorated during bare fiber irradiation at ∼910 °C. However, fiber strength deterioration was not observed when irradiated in composite form. Irradiation effects on the fiber-matrix interface properties were discussed based on results from the composite and single filament tensile tests, the hysteresis analysis, and the fracture surface examination.