Microstructures in high purity β-SiC irradiated with fast neutrons (up to ∼9.6 × 10 25 n/m 2, in HFIR) at very high temperatures (1130, 1300, and 1460 °C) were studied by transmission electron microscopy. Cavities and dislocation loops were generally observed in irradiated samples. The cavities were preferentially formed at grown-in stacking faults, and were spherical in shape below 1300 °C and mainly faceted with {1 1 1} planes at 1460 °C. Estimated volume fractions of observed cavities were much smaller than macroscopic densitometer swelling recently reported, which implies other defects cause the swelling in this temperature regime. Larger Frank loops (>25 nm in radius) formed at 1460 °C were identified as interstitial type using the inside/outside method. Unfaulting of the loops was not observed or was very rare. Rapid loop growth and density decrease were observed in the temperature range of 1300–1460 °C concurrently with the rapid cavity growth. The limited growth rate of dense loops at lower temperature was discussed in terms of high sink density estimated from a grain-boundary-loop-denuded zone formed at 1130 °C.