The ultrasonic phased array technique is one of the most effective tools for visualizing the flaws in structural components. However, in the case of flaw size measurement, only an A-scope signal is usually used. On the other hand, the Time-of-Flight Diffraction (TOFD) method is known as the most accurate flaw sizing method. This method is, however, only applicable if the flaw exists on the perpendicular bisector of the ultrasonic probe pair. In this study, a flaw sizing method, based on the least squares method using multiple ultrasonic probes, is proposed. In order to identify the flaw tip position accurately, the delay time of the flaw tip echo must be taken into account. The proposed method can compensate for the delay time as an unknown parameter. The effectiveness of the proposed method is investigated by numerical simulation. It is shown that the flaw tip position can be identified by the proposed method, though the identification error is affected by the number of ultrasonic probes. In addition, the relationship between the probe arrangement and measurement error can be evaluated using the dilution of precision value.

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