The microstructural evolution of SiC/SiC composites after Si2+ with/without He+ ion irradiation was studied using transmission electron microscopy. The temperature, displacement damage level, and He/dpa ratio were 1273/1673K, 10/100dpa and 0/60appmHe/dpa, respectively. In 10dpa single-ion irradiation, no cavity was detected at 1273 and 1673K. But cavities were observed locally at 1673K, 100dpa. In dual-ion irradiation, cavities were observed at 1673K, 100dpa. Helium bubbles (d<5nm) were formed densely on {111} faulted planes in the fiber and matrix. And lens-shaped cavities (major axis 2a=20-50nm) were formed on grain boundaries in the matrix. The swelling by cavities in CVI matrix is about 0.5% at 80dpa and 0.7% at 130dpa. Loss of PyC layer beneath the irradiated surface was observed (single-ion: about 500nm, dual-ion: about 1 m). And the thickness of the PyC layer expands after single/dual-ion irradiation (single-ion: 12%, dual-ion: 29% increase). But Tyranno-SA/PyC/CVI composites shows showed better microstructural stability than expected at 1673K.