In a method of obtaining a gum impression of a patient's mouth and measuring jaw relations, a dental impression tray assembly including a lower tray and an upper tray to fabricate a denture, is used. The lower tray includes a first piece and a pair of second pieces and the upper tray includes a third piece and fourth piece. The method includes inserting the lower tray loaded with an impression material into the mouth to take an impression; taking out the lower tray from the mouth and cutting the impression material along a borderline between the first piece and the pair of second pieces; separating the first piece from the pair of second pieces; attaching an intra-oral tracer to the first piece and inserting the first piece, to which the intra-oral tracer is attached, and the third piece into the mouth to measure the jaw relations.