Disclosed is a method for appointing an orientation flat, in which when there are a plurality of orientation flats with the same length, any one from among the orientation flats of the same length can be certainly appointed as a reference orientation flat. In the disclosed method, three orientation flats at three positions are detected, respectively, through the rotation of a semiconductor wafer, the lengths of three circular arcs between the three orientation flats are obtained, respectively, and then an orientation flat at the right side of the longest circular arc from among the three circular arcs is appointed as a reference orientation flat. Accordingly, it is possible to certainly appoint a reference orientation flat even though there exist a plurality of circular arcs with the same length.