Structure of seat back is provided, which comprises: a seat back frame; and an upholstery cover assembly including a forward upholster cover element and a backward upholstery cover element. Those two upholstery cover elements cover respective forward and backward sides of the seat back frame, with an upper region thereof secured on top portion of the seat back frame by a headrest stay holder element. Outer peripheral ends of those two upholstery cover elements are engaged together by an engagement element, so that substantially a whole of the seat back frame is covered with the upholstery cover assembly, thus forming a seat back.