In this paper, the toughening of zirconia ceramics with dispersed silicon carbide nano-particles is studied.Based on both the experimental observations, three effects of nano-particles on the toughness of nano-composite ceramics, namely, nano-particle clustering, crack pinning and transgranular fracture, are identified from both the experimental and analytical studies.And a model considering the above toughening effects is developed to predict the overall toughness of nano-composite ceramics.