Abstract：An experimental apparatus to test the rock fracture toughness values of model III fracture is designed；and the new kind of rock sample adapted to the apparatus is also designed. A series of stable fracture toughness values of model III fracture in granite and marble are achieved by the experiments. At the same time，the fracture toughness values of model III fracture in cast iron and mild steel are also obtained. The comparison between test results of the two kinds of materials is given. According to the analysis with fracture mechanics and finite element method，the stress fields near mold III fracture tip are obtained. The relationship of vs. the crack depth d is achieved. Thus，the reasons that the fracture surface varies from helicoids to plane shearing surface with the crack depth d aggrandizing gradually in model III fracture are revealed.
