The present invention relates to a graft copolymer having excellent thermal stability, a method of preparing the graft copolymer, a thermoplastic resin composition including the graft copolymer, and a molded part including the thermoplastic resin composition. According to the present invention, since the thermoplastic resin composition of the present invention includes the graft copolymer having excellent thermal stability, the thermoplastic resin composition and the molded part manufactured using the thermoplastic resin composition have excellent physical properties, such as impact strength, fluidity, and transparency. Furthermore, the degree of discoloration that may occur when the resin composition or the molded part is allowed to stay in an injection molding machine or stay at a high temperature for a long period of time may be reduced.