As material flaws have great influence on fatigue strength of cast materials components, it is important to establish fatigue strength evaluation method of cast materials with flaws. In this study, concept of allowance rate was introduced to evaluate fatigue strength of cast steel with round-shaped flaws. The results obtained were summarized as follows : (1) A fracture mechanics approach was made to estimate fatigue limit of cast steel with a round-shaped flaw, regarding the flaw as an elliptical notch having an intrinsic crack, which is material constant, at the root. By this method, the allowance rate of flawed body againtst cracked one can be estimated. (2) The allowance rate can also be obtained experimentally by brittle fracture test of notched body at an extremely low temperature. (3) The concept of allowance rate was verified by fatigue test of specimen with a notch and with an actual casting flaw.