Tungsten Carbide-Cobalt (WC-Co) nanoparticle composites have wide applications because of their hardness and toughness. In this study, apoptosis and related signaling induced by WC-Co were investigated. Electron spin resonance (ESR) and fluorescent staining indicated that both WC-Co nano- and fine particles stimulated reactive oxygen species (ROS) generation. Catalase inhibit WC-Co-induced ROS as well as mitochondrial membrane permeability damage, indicating that H2O2 may play an important role in the cytotoxicity induced by WC-Co. Further studies indicated that WC-Co nanoparticles elicited a higher cytotoxicity and apoptotic induction than fine particles. Western-blot analysis showed an activation of proapoptotic factors including Fas, FADD, caspase 3, 8 and 9, BID, and BAX. Lamin A and beta-actin were cleaved. Interestingly, WC-Co particles also induced Bcl-2, an anti-apoptotic factor, up-regulation. In addition, both cytochrome c and AIF were up-regulated and released from mitochondria to the cytoplasm. The results demonstrate that apoptosis induced by WC-Co involve both 'extrinsic' and 'intrinsic' apoptotic pathways and nanoparticles exhibit higher cytotoxicity and apoptotic induction than fine particles.