A main object of the present invention is to provide a receptor layer forming resin which makes it possible to obtain a receptor layer for a thermal transfer image receptor sheet having excellent dyeing property. The present invention achieves the above object by providing a receptor layer forming polyurethane resin for a thermal transfer image receptor sheet obtained by copolymerizing: a high molecular weight polyol having a number average molecular weight of 1000 or higher (A); an organic isocyanate (B); and according to need, a polyol compound having a number average molecular weight of 500 or lower (C), wherein the receptor layer forming polyurethane resin for a thermal transfer image receptor sheet meets the following requirements (1) to (4): (1) the resin has at least two glass transition temperatures; (2) at least one of the glass transition temperatures is between -200 to 0.degree. C., and at least one is between 20 to 100.degree. C.; (3) the high molecular weight polyol (A) is a mixture comprising; an aromatic polyester polyol containing metallic base sulfonate (A-1); and a polyol other than said (A-1) (A-2); and (4) the ratio of (A-2) in the high molecular weight polyol (A) is 20 mass percent or higher.