The present invention relates to a method of preparing a graft copolymer, and a thermoplastic resin composition including the same. More specifically, the present invention relates to a method of preparing a graft copolymer, the method including a step of preparing graft copolymer latex by graft-polymerizing 20 to 40 % by weight of an aromatic vinyl compound and 1 to 20 % by weight of a vinyl cyanide compound onto 50 to 70 % by weight of conjugated diene rubber latex having a solids content of 30 to 65 % by weight; and a step of coagulating the prepared graft copolymer latex by adding 0.04 to 0.27 parts by weight of a nonionic polymer coagulant as a first coagulant, and 0.47 to 1.65 parts by weight of a cationic polymer coagulant or 0.73 to 3.35 parts by weight of an anionic polymer coagulant as a second coagulant to 100 parts by weight (based on solids) of the prepared graft copolymer latex, wherein, when the second coagulant is the cationic polymer coagulant, the total weight of the nonionic polymer coagulant and the cationic polymer coagulant is 0.7 to 1.7 parts by weight, and when the second coagulant is the anionic polymer coagulant, the total weight of the nonionic polymer coagulant and the anionic polymer coagulant is 0.97 to 3.37 parts by weight, and a thermoplastic resin composition including the graft copolymer. According to the present invention, when a graft copolymer is prepared using the above-described method, the graft copolymer may have improved coagulation efficiency and high gloss. In addition, a b-value measured using a Hunter lab colorimeter and a b-value measured after being left may be small, thereby realizing excellent color properties.