Abstract: It presents a theoretical model and its associated analytical expression for determining the fracture toughness of concrete and rock using unnotched specimens. The important influence of the size of concrete aggregates and rock particles is fully considered in the proposed model, and only the peak load of small-scale unnotched specimens of concrete or rock is required. Size-independent fracture toughness can be directly determined. Furthermore, a series of fracture tests on the rock specimens with different notch lengths is conducted. Experimental results show that the fracture toughness of unnotched rock specimens obtained using the proposed model is consistent with that determined from the prefabricated specimens with a notch of 1 mm length. The fracture toughness of unnotched specimens is also consistent with that of the notched specimens based on the regression analysis method. The test results of the unnotched and notched rock and concrete specimens from other studies are further analyzed using the proposed model. The rationality and applicability of the proposed model and associated method are also verified. It provides new insights into the determination of the size-independent fracture toughness of concrete and rock by using small-scale unnotched specimens in a laboratory.