The present invention relates to an ABS graft copolymer, a method of preparing the same, and a thermoplastic resin composition including the same. More specifically, when an ABS graft copolymer is prepared using emulsion polymerization, a phosphate ester-based reactive emulsifier capable of acting as a metal deactivator is added, thereby reducing the amount of undesired residues remaining in the ABS graft copolymer latex. As a result, thermal stability may be improved without adding a heat stabilizer. In addition, compatibility between the ABS graft copolymer and the aromatic vinyl compound-vinyl cyanide compound copolymer may be improved by the phosphate ester-based reactive emulsifier. Therefore, a final molded part having improved impact resistance, e.g., increased impact strength, and having improved appearance quality, e.g., excellent whiteness, while having excellent thermal stability may be prepared.