The brittle fracture initiation characteristics of circumferentially notched round bar, CNRB, subjected to the axial load have been investigated theoretically and experimentally using JIS-SCM 440 and SMn 443 steel bars. The curve showing the relationship between net section stress at fracture, a and temperature, T, has two regions macroscopically. One is called Region I where σn, F diminishes with decreasing T, and the other is Region II where σn, F is nearly constant irrespectively of T. The effect of diameter, D, and notch depth, c, on σn, F in Region I can be evaluated on the basis of the linear elastic fracture mechanics. σn, F at the transition temperature from Region I to II, σn, TR, is a little higher than the general yieding stress level of CNRB analyzed with the aid of finite element method. σn, TR/σY (σY: yield stress) shows little change with D and strength of materials, and rises with an increase in relative notch depth, 2c/D. σn, F in Region II can be approximately estimated from the experimental equation relating between σn, TR/σY and 2c/D. Finally, a procedure is proposed to evaluate the brittle fracture strength of CNRB subjected to axial load.