In the previous report, we reported the phenomenon in which strain rate-temperature paramerter (R) keeps some constant value in Intensely Deformed Nonlinear Zone (IDNZ) named by J. R. Rice in the vicinity of a crack tip.This phenomenon means that we might need not consider the initiation point of brittle fracture in case of assessing the quantitative effect of strain rate on fracture toughness. Many fracture toughness test results give us that fracture toughness (critical CTOD and J integral) is the function of yield stress at fracture initiation point. And yield stress is the function of strain rate -temperature parameter (R). Then we might expect that fracture toughness is the function of R value in IDNZ. In this paper, we investigate the relation between R value in IDNZ and fracture toughness. As a result of this investigation, it turns out that fracture toughness is the function of R value in IDNZ. As we know that fracture toughness is the function of R value in IDNZ, we try to estimate fracture toughness at arbitrary crosshead speed by using thermo elasto-plastic FEM. As a result of this estimation, it becomes clear that we can get the correct value of fracture toughness on the theory of this report. Moreover fracture toughness is considered to be affected by strain rate because of high strain concentration even if it is quasi-static condition and this is one of causes of having large scatter in fracture toughness.