The characteristics of both fracture stress curves, cleavage and fibrous were investigated in regard to their dependences on temperature and stress-triaxiality. Used for experiments are a killed mild steel and high tension steel. As the results of the experiments, the following important informations are clarified. (1) The cleavage fracture stress curve is hardly affected by changes in temperature and stress-triaxiality as shown in Fig. 4. (2) The fibrous fracture stress curve is remarkably sensitive to those changes as shown in Figs. 7 & 9. (3) The condition for fibrous fracturing is identically represented by one stress curve regardless to the degree of stress-triaxiality, in case where the octahedral shearing stress and strain (τ_oct and γ_oct) are used, as shown in Fig. 11.