A criterion of the transition from stable fibrous crack growth to unstable fracture is investigated on the basis of J -integral R curve concept. The slope of J_R curve is compared with that of crack driving force curve under fixed grip condition. Instability condition is expressed by dJ_R / da ≤ dJ^F.G. / da And the expression of above formula for bending specimen is obtained which is a general form of the expression after Paris et al. Three-point bending tests with spring bars were carried out at temperatures above brittle-ductile transition. Instability occurred after stable fibrous crack growth at the lower level of J value than conventional bending tests. The proposed instability criterion compares well with the test results.