The relationship between the EFBH (Equivalent Flat-Bottom Hole) size measured by non-destructive method and the actual size by destructive method in many rotors manufactured at Doosan Heavy Industries & Construction Co. Ltd. was investigated. In this investigation. "the Master Curve" compensating the differences between UT reported sizes and actual sizes of defects in our rotor forgings were obtainable. The applicability of this "Master Curve" as a way of calculating the actual defect size was also investigated. For the evaluation of rotor forgings, it is expected that this "Master Curve" may be used to determine the accurate actual sizes of defects.