The thesis is focused on the prediction of the fracture toughness temperature dependence through a universal curve of fracture toughness (also known as the master curve). To determine the parameters of the universal curve of fracture toughness, values acquired from the measurement results of fracture toughness and tensile tests of structural steel P91 are used. The theoretical part is based on a summary of the relevant information from the field of fracture mechanics and brittle-ductile fracture behavior of steels that are important for the understanding of fracture-strain response of materials depending on load conditions. The experimental part of the thesis contains the results from practical measurements and analyses, which were used for determining the parameters of a universal curve of fracture toughness as well as for the evaluation of fracture behavior and description of the impact of structural parameters on this behavior in case of steel P91.