Competition authorities move away from the practice that mainly depend upon market structure indicators such as the sum of the company’s market share or its increase in reviewing mergers, and instead, tend to examine their anti-competitiveness by also analyzing various factors related to the competition situation in relevant markets other than market structure indicators. Hence, the theory of maverick firm is expected to play an important role in reviewing horizontal mergers in the future. The theory of maverick firm is used as a basis for competition authorities to hold mergers anti-competitive even if significant increase in the market concentration is not caused by the mergers. However, discussions on this theory have been scarce internationally and moreover, in Korea, discussions have not been made both theoretically or practically until filing of mergers to acquire CJ Hello in 2015 and 2019. Due to its nature, the application of the theory may inevitably create risk of impairing the objectivity and predictability of the judgment of anti-competitiveness of horizontal mergers. In order to secure objectivity and appropriateness of merger reviews applying the theory of maverick firm, the Guidelines for Combination of Enterprises Review should be amended to provide for the concept of maverick firm and the criteria for identifying maverick firms, theories of anti-competitive harms based on this theory. In addition, the Korea Fair Trade Commission should also comprehensively analyzes in reviewing mergers whether there is a business entity playing a role of maverick firm among other competitors in the relevant markets as well as the merging parties. Moreover, it is necessary to analyze in detail what changes will be caused by the merger to the ability and incentives for the maverick firm to compete and whether third-party competitors, who were previously unable to act as maverick, will have the ability and incentives to perform the role of maverick through the merger.