The construction of the relation between the critical energy release rate, $G_{Ic}$, and the mode I fracture toughness, $K_{Ic}$, is of great significance for understanding the fracture mechanism and facilitating its application in engineering. In this study, fracture experiments using NSCB and CCCD specimens were conducted. The effects of specimen sizes, loading rate and lithology on the relation between $G_{Ic}$ and $K_{Ic}$ were studied. $G_{Ic}$was calculated by integrating the load–displacement curve according to Irwin’s approach. Based on the measured $K_{Ic}$ and $G_{Ic}$ of the rock specimens, a relation between $G_{Ic}$and $K_{Ic}$ was found to be different from the classical formula under linear elasticity. It was found that both specimen size and loading rate do not influence this relation.