A method for manufacturing a thermoplastic composition including: a polyester resin (c); a mixture including a copolymer (a) of an α-olefin and of a monomer having an ethylenic unsaturation and an epoxy function, as well as a core-shell compound (b); characterized in that said method includes a first step of manufacture the mixture of (a) and (b) by extrusion at a temperature at which the copolymer (a) is in the molten state and at a maximum temperature of 60 to 180° C.; a second step of manufacturing the thermoplastic composition by extrusion or by mixing the polyester resin (c) with the mixture of (a) and (b) produced in the first step.