Abstract: The study of size effect on the fracture toughness of nuclear grade graphite IG-11 by using extended finite element method (XFEM) which is based on the cohesive zone model (CZM) to simulate the fracture of single-edge-notched beams under three-point-bending was carried out. The influence of specimen size and thickness was studied both in the experiment and the simulation. Besides, sensitivity analysis of the fracture parameters was under consideration in the numerical analysis. The predicted fracture toughness is in a range of 0.90-1.10 MPa•m^1/2 which is close to the experiment result of 0.82-1.27 MPa•m^1/2. And the simulation results show that crack initiation strength has greater influence on the numerical analysis than the fracture energy of the material. In addition, the fracture toughness of nuclear graphite shows a size effect, i.e. the fracture toughness increases as the specimen size increases and finally converges to a constant value, which agrees with the experiment and the prediction from recent size effect model as well. However, the specimen thickness has no obvious effect on K_IC.