It is well known that fracture toughness value is affected by strain rate. As for strain rate, the dependency of fracture toughness on fracture parameter rate has been usually studied. In the linear fracture mechanics, fracture parameter rate ( dK/dt ) is directly related to strain rate. However, in the non-linear fracture mechanics, the relation between fracture parameter rate ( d δ/ dt , dJ/dt ) and strain rate has not yet been made clear. Therefore, in the safety assessment of real structures, non-linear fracture parameter can not be applied in case of which the effect of the strain rate on the fracture toughness needs to be considered. In this paper, fracture initiation point is assumed to be the tip of the intensely deformed non-linear zone defined by Rice, which is considered to show the maximum triaxial constraint. Then the strain rate at the supposed fracture initiation point is given in connection with remote strain rate which corresponds to the strain rate in non-cracked plate. By use of the newly difined strain rate, strain rate-temperature parameter is determined, which represents the effect of strain rate on fracture toughness. Compact tension tests and COD tests with different strain rate and temperature, as well as K _Id tests are carried out to verify above conception.