Fracture toughness is an important mechanical property for designing structures or for conducting safety assessments. In this study, determination methods of fracture toughness of P91 material were studied using finite element simulation based on the Gurson-Tvergaard-Needleman (GTN) model. Results indicated that the JIC of P91 could be exactly determined using the simulation of standard fracture toughness test based on the GTN model. When the small punch test (SPT) was used to determine JIC, a notched-plate specimen provided better results to determine the JIC of P91 compared to the other two methods. A notched-tube specimen of SPT was proposed to determine JIC of nuclear reactor fuel cladding tubes of P91. Simulation results indicated that the notched-plate specimen and the notched-tube specimen provided identical accuracy for determining JIC. Therefore, the determination method derived from the notched-plate specimen could be used to the notched-tube specimen.