A dense oxidation protective SiC-Si ceramic coating with well-dispersion SiC nanowires was prepared on carbon/carbon (C/C) composites using a two-step technique consisting of chemical vapor deposition and hot-pressing reactive sintering. The results showed that the toughness and thermal shock resistance of the coating were dramatically improved, resulting in an excellent oxidation protection for the coated C/C composites, which is attributed primarily to the toughening effects of SiC nanowires in the coating including two special energy dissipation mechanisms involving the super-plasticity deformation of SiC nanowires and the plastic fracture at the interface of the nanowire-matrix, except for nanowire pullout, nanowire bridging, and microcrack deflection. More importantly, a novel oxidation protective failure mechanism of the coated C/C composites revealed that the presence of Al3+ in the coating, damaged the network structure of SiO2 glass and resulted in the slight oxidation of C/C composites.