The present invention relates to a thermoplastic resin composition, a method of preparing the same, and a molded part manufactured using the same. More specifically, the present invention provides a thermoplastic resin composition having excellent mechanical properties, such as tensile strength and impact strength, and weather resistance while having low gloss characteristics, a method of preparing the same, and a molded part manufactured using the same. According to the present invention, the thermoplastic resin composition includes an ASA graft copolymer having a core-shell structure having an average particle diameter of 0.05 to 1 μm, an aromatic vinyl compound-vinyl cyanide compound copolymer, and an epoxy group-containing copolymer having a weight average molecular weight of 2,000 to 8,000 g/mol in specific amounts.