The thermomechanical durability of a two-dimensionally woven and a three-dimensionally braided SiC/SiC composite fabricated by chemical vapour infiltration technique were studied. The effects of thermal shock, thermal ageing, and thermal cycling at high temperatures on mechanical properties were determined. The thermal degradation mechanisms were also investigated. The results indicated that the fibre geometry, matrix porosity, interlaminar shear strength and testing conditions affect the thermomechanical durability of the SiC/SiC composites.