A new method for the determination of fracture toughness of body-centered cubic (BCC) metals and alloys of which the yield strengths are very susceptible to temperature and deformation rate is proposed. These materials also change their energetic and deformation characteristics at a certain temperature. The hypothesis states that the fracture toughness of BCC metals and alloys are exponentially related to the thermoactivation energy of plastic deformation at the crack tip. The theoretical basis of the study depends on two observations: (1) the difference of athermic and thermoactivation zones in the yield strength vs. temperature diagram of mentioned alloys and (2) the crack propagation mechanism with the microcracks developed at some distance ahead of the crack tip under a critical stress magnitude and then propagating to the main crack to grow it. The relations obtained between fracture toughness, basic mechanical properties and microstructure of these alloys have given a good agreement between the results of present study and the results of experiments according to ASTM standard.