Microstructural evolution of silicon carbide fiber-reinforced silicon carbide matrix composites (SiC/SiC composites) during dual-beam ion irradiation was studied. The composites had been fabricated by means of chemical vapor infiltration (CVI) and polymer impregnation and pyrolysis (PIP) methods using advanced fibers, i.e., Hi-NicalonTM Type-S and TyrannoTM-SA, as well as conventional fibers. Dual-beam ion irradiation was performed to 10 dpa at 873K and 60appmHe/dpa. Composites with advanced low-oxygen near-stoichiometric SiC fiber with high crystallinity and beta-SiC matrix exhibited superior irradiation resistance, in contrast to that amorphous Si-C-O based fibers and conventional polymer-derived matrix exhibited microstructural instability in association with irradiation-induced crystallization. Pyrolytic carbon deposited as the fiber-matrix (F-M) interphase exhibited irradiation-induced amorphization and the helium co-implantation enhanced it. Post-irradiation heat treatment caused significant microstructural changes across the F-M interphases at temperatures as low as the composites are supposed to retain their mechanical strength.