A thermal ink ribbon has a base sheet, a thermal recording layer arranged on one side of the base sheet, and a back layer arranged on an opposite side of the base sheet. The back layer according to the present invention comprises a polyurethane resin which is obtained by reacting a low molecular weight polymer having at least one active-hydrogen-containing group at one end of the molecule, a compound having at least one active-hydrogen group and at least one hydrophilic group except a hydroxide group, with a polyisocyanate. The back layer is excellent in the adhesiveness to a base sheet, the heat resistance relative to a thermal head and slipperiness to a thermal head; and the ink ribbon with such a back layer has no problems in printed characters and figures on printing, in the migration of impurities from the back layer into an ink layer, in smeariness of a thermal head and in its maintenance. Such a back layer is capable of being formed on a surface of a base sheet by only applying the coating, which comprises the above polyurethane resin, for forming a back layer; and drying the coated layer.