The hardness and elastic modulus of monolithic β-SiC and advanced SiC/SiC composites irradiated to He-concentrations of from 1000 to 20 000 at.ppm at a temperature below 100°C were investigated. Changes in the hardness and elastic modulus were correlated with He release behavior and with irradiation-induced microstructural changes and swelling. The hardness and elastic modulus of the irradiated specimens decreased with He concentration. Amorphized regions were observed in the He-irradiated monolithic β-SiC. The mechanical property changes likely were due to irradiation-induced amorphization. By the annealing at 1200°C, nearly full recovery was achieved. He release peaks from the pyrolitic graphite, monolithic SiC, and the SiC/SiC composites at 250, 700, 1000, and 1150°C, were identified. The correlation between the mechanical property and microstructural changes and the He release behavior are discussed.