Disclosed is a method of surface treatment of a thermoplastic resin film (i) which comprises the steps of subjecting the thermoplastic resin film (i) to surface oxidation, coating the oxidized film with a surface modifier (A) and then stretching the coated film, and which is characterized in that: the surface modifier (A) is an aqueous dispersion prepared by dispersing an olefin copolymer (a) with an unsaturated carboxylic acid or its anhydride bonded thereto in water along with at least one dispersant (b) selected from the group consisting of nonionic surfactants, nonionic water-soluble polymers, cationic surfactants and cationic water-soluble polymers; the ratio by weight of (a)/(b) in terms of the solid content thereof falls between 100/1 and 100/30; and the mean particle size of the resin in the surface modifier (A) is at most 5 nullm. The method can provide a thermoplastic resin film of good melt thermal transfer printability and good offset printability capable of giving prints of good water resistance.