Abstract - Vickers test method and many equations presented by researchers are used for determining the fracture toughness of brittle materials.These equations are generally based on the relationship between the crack lengths around the indentation zone of Vickers test and the fracture toughness in the specimen. There is only one equation including a semi-empirical coefficient based on the indentation surface and the fracture toughness of the specimen. In this paper, improvement the accuracy of semi-empirical coefficient in this equation is studied for determining the fracture toughness of specimens without additives and containing 5 wt% phenolic resin experimentally. Increasing the accuracy of semi- empirical coefficient leads the increasing the extent of application and accuracy of the results of fracture toughness obtained from the equation. The accuracy of fracture toughness equation coefficients semi- empirical coefficient from 0.003693 to 0.003655 arrived. This equation has minimal cost of experiments for determining fracture toughness of different brittle materials.