A new method to estimate the flow profile correction factor (FPCF) for a transit-time ultrasonic flow meter (UFM) having a diametral transducer configuration is introduced in this work. For the adaptation of a diametral UFM for a feedwater measurement, the optimized flow profile correction factor is obtained through experiments and a simulation in actual flow conditions. The log function curve fitting is performed on a combined data set; the velocity ratio of UFM reading versus standard fluid measurement at low flow velocity, UFM reading versus standard fluid measurement at medium flow velocity, and UFM reading versus clean Venturi measurement at high flow velocity. Through an uncertainty analysis, the uncertainty of the FPCF is calculated. The resultant uncertainty of the new FPCF is 0.335%. This value is approximately half the value presented by UFM manufacturers. By adaptation of a diametral UFM with the new FPCF proposed in this work, feedwater flow measurement in nuclear power plants (NPPs) can be easily performed at lower cost than either chordal or cross-correlation UFM.