A coil forming method, a coil forming die assembly and a coil manufactured by the coil forming method are disclosed. The coil forming method comprises conducting preliminary forming work on a coil under a linear state before conducting bending work on the coil. The preliminary forming work including conducting compression work on the coil under the linear state to compress the coil in a region covering a rounded corner portion formed when bending work is conducted to decrease a dimension of the coil in a widthwise direction. The dimension of the coil is decreased in the widthwise direction by the nearly same dimension as that of the coil bulged in the widthwise direction when bending work is conducted in the absence of the preliminary forming work. This prevents the occurrence of bulging of the coil during bending work with an increase in lamination factor.