Microstructure and mechanical/thermal properties of SiCf/SiC composites additionally reinforced with SiC nanowires were investigated. SiC nanowires could be grown successfully within the SiC fiber preform by a chemical vapor infiltration (CVI) process through a control of the deposition parameters. The incorporation of the SiC nanowires into the fiber perform was clearly shown to be effective in increasing the efficiency of the matrix infiltration, and thus resulted in a higher density in a shorter CVI time. The modification of the pore structure and the reduction of the macro-pores in the composite resulted in higher mechanical and thermal properties than the conventional CVI counterpart. The matrix deposition on both surfaces of the SiC fibers and nanowires induced smaller matrix grains in the nanowire-reinforced composite, and thus a higher hardness and elastic modulus than the conventional one after an ion irradiation.