The present invention relates to a thermoplastic resin composition including a (meth)acrylate graft copolymer and a method of preparing the thermoplastic resin composition. According to the present invention, the thermoplastic resin composition includes a graft copolymer (A) containing an alkyl acrylate rubber core and an alkyl methacrylate compound (co)polymer shell surrounding the alkyl acrylate rubber core; and a matrix resin (B). In this case, the rubber core has an average particle diameter of 40 to 100 nm, the graft copolymer (A) has a grafting degree of 20 to 100%, and the thermoplastic resin composition has a refractive index of greater than 1.46 and less than 1.49 and a blackness (L value) of less than 25.0.