The tensile properties and fracture behavior of polyacrylonitrile (PAN)- and pitch-based hybrid carbon fiber/polyimide composites with several types of nanoparticles (25 nm C, 20-30 nm beta-SiC, 130 nm beta-SiC, 80 nm SiO2, and 300 nm SiO2) added to the matrix were investigated. The tensile stress-strain curves of PAN- and pitch-based hybrid carbon fiber/polyimide composites with 25 nm C, 20-30 nm beta-SiC, and 80 nm SiO2 nanoparticles have complex shapes (jagged trace), whereas the tensile response of hybrid carbon fiber/polyimide composites with 130 nm beta-SiC and 300 nm SiO2 nanoparticles indicates an instantaneous failure. The stress after the initial failure in hybrid carbon fiber/polyimide composites improves by adding 25 nm C, 20-30 nm beta-SiC, and 80 nm SiO2 nanoparticles to the matrix and correlates with the fracture toughness of the polyimide matrix.