The invention relates to a method for forming a mouth roll (2) to a cup (1) made of plastic-coated board and to the cup provided with a mouth roll produced by the method. The mouth roll for stiffening the cup is produced with a suitable striking tool bending the board at the mouth of the cup. In accordance with the invention, the mouth roll is additionally stabilized by melting the plastic coating (8) of the board under heat so that the opposite surfaces of the board adhere to each other by heat-sealing in the area of the roll. Melting can be performed e.g. by hot-air blasting directed to the mouth roll at is production stage.