Abstract: Based on the fracture mechanics theory under confining pressure conditions, the authors test rock fracture toughness using the thick-wall cylinder specimens with double-wing fractures and different clay contents under various confining pressures. The authors then use the finite element model to interpret the toughness results. The regression analysis based on a number of experiments indicates that the relationships of fracture toughness with confining pressure and shale contents are linear with small errors. The experimental procedures and interpreting method presented in this paper assure a better accuracy for determination of fracture toughness for hydraulic fracturing than that obtained from experiments without confining pressures, and are thus favorable for improvement of hydraulic fracturing performance.