Disclosed are a thermoplastic resin composition, a method of preparing the same, and a molded part manufactured using the same, wherein the thermoplastic resin composition includes a-1) 1 to 30% by weight of a first graft polymer obtained by graft-polymerizing an aromatic vinyl compound and a vinyl cyanide compound onto a conjugated diene rubber having an average particle diameter of 0.05 μm or more and less than 0.2 μm; a-2) 5 to 45% by weight of a second graft polymer obtained by graft-polymerizing an aromatic vinyl compound and a vinyl cyanide compound onto a conjugated diene rubber having an average particle diameter of 0.2 to 0.5 μm; b) 50 to 80% by weight of an aromatic vinyl compound-vinyl cyanide compound copolymer; and c) 1 to 10% by weight of a (meth)acrylic acid alkyl ester polymer.