The plastic work concentrated around a crack tip yields considerable heat and then the heat raises the temperature around a crack tip at high loading rate because there is not enough time to release heat. The temperature rise has influence on fracture toughness and then the quantitative grasp of the temperature rise distribution near a crack tip due to plastic work is necessary to evaluate the quantitative effect of strain rate on fracture toughness for high toughness steels. In this report, CT tests were carried out under various crosshead speeds and the temperature rise distributions along the crack line were measured during loading process by a thermo-viewer. Moreover tensile tests for round bar specimens were also carried out under various crosshead speeds for the purpose of surveying the convertible ratio of plastic work into heat. Then the dynamic thermal elasto-plastic finite element analysis which was already reported was performed under the condition of CT tests. The analytical temperature rise distribution near the crack tip showed the good agreement with the experimental one.The analytical results also gave the important clue to establish the effect of strain rate on fracture toughness, that is, the phenomenon that yield stress fairly keeps constant in the region of Intensely Deformed Nonlinear Zone named by J. R. Rice in the vicinity of a crack tip because of opposed effects of strain rate and temperature on yield stress. In other words, this phenomenon has a significant meaning that strain rate-temperature parameter is expected to keep some constant value in IDNZ and we might need not consider the initiation point of brittle fracture when invesigating the effect of strain rate on fracture toughness for high toughness steels.