In this paper, micrometers long and 20–100nm diameter SiC nanowires had been synthesized in the Cw/SiC composites by annealing treatment. The present work demonstrated that it is possible to fabricate in-situ SiC nanowires toughening Cw/SiC composites. La2O3 had a significant effect on the morphology and distribution of the SiC nanowires in the Cw/SiC composites. The excess lanthanum resulted in the growth of SiC nanowires with an increasing growth rate in the longitudinal direction and a decreasing growth rate in radial direction. The addition of La2O3 would improve the mechanical properties in the room-temperature. The vapor–liquid-solid growth mechanism of the SiC nanowires along <111> direction was proposed. The combination of grain bridging, crack deflection, whisker debonding and SiC nanowires can improve fracture toughness.