Technology of making freestanding gallium nitride (GaN) wafers has been matured at length. Gallium nitride is rigid but fragile. Chamfering of a periphery of a GaN wafer is difficult. At present edges are chamfered by a rotary whetstone of gross granules with weak pressure. Minimum roughness of the chamfered edges is still about Ra 10 .mu.m to Ra 6 .mu.m. The large edge roughness causes scratches, cracks, splits or breaks in transferring process or wafer process. A wafer of the present invention is bevelled by fixing the wafer to a chuck of a rotor, bringing an edge of the wafer into contact with an elastic whetting material having a soft matrix and granules implanted on the soft matrix, rotating the wafer and feeding the whetting material. Favorably, several times of chamfering edges by changing the whetting materials of smaller granules are given to the wafer. The chamfering can realize small roughness of Ra 10 nm and Ra 5 .mu.m at edges of wafers.