Abstract: The present invention provides a process for preparing thermoplastic resin compositions having excellent HCFC resistance which comprises:a first step wherein 50 to 90 parts by weight of a monomer mixture of 50 to 80% by weight of a vinyl cyanide, 20 to 50% by weight of an aromatic vinyl compound and 5% by weight or less of an unsaturated carboxylic acid or esters thereof or a vinyl acetate are graft-polymerized in the presence of 10 to 50 parts by weight of a rubber latex and also in the presence of 0.001 to 2.0 parts by weight of a polymerization inhibitor to prepare a graft polymer(A);a second step wherein 100 parts by weight of a monomer mixture of 50 to 80% by weight of a vinyl cyanide, 20 to 50% by weight of an aromatic vinyl compound and 5% by weight or less of an unsaturated carboxylic acid or esters thereof are polymerized, while adding 2.