An object of the present invention is to provide a paint composition for in-mold coating of thermoplastic plastic substrates, the paint film resultant from which composition neither peels off nor becomes turbid even if the mold is opened in a state where its temperature is high, needless to say that the resultant paint film has excellent adhesion to nonpolar thermoplastic plastic substrates and adequate flowability. As a means of achieving this object, a paint composition according to the present invention for in-mold coating of thermoplastic plastic substrates comprises: an acryl-modified unsaturated dicarboxylic acid (anhydride)-grafted polyolefin (A); at least one member (B) selected from the group consisting of reactive oligomers and reactive monomers; and a radical polymerization initiator (C); in a specific formulation ratio, with the paint composition being characterized in that: the acryl-modified unsaturated dicarboxylic acid (anhydride)-grafted polyolefin (A) is in the range of 92 to 112.degree. C. in melting point and in the range of 5 to 15 mass % in ratio for which unsaturated dicarboxylic acid (anhydride)-derived structural units account in its structure.