The Monopoly Regulation and Fair Trade Act in Korea prohibits mergers which substantially restrict competition in the line of commerce. The term "the act substantially restrict competition" refers to any of the practices that affect or threaten to affect the setting of price, quantity, quality, or other terms or conditions of trading in accordance with the intent of a certain enterpriser or an enterprisers' organization owing to reduced competition in a line of commerce. Market concentration, which is found based on market share of the company is a key indicator in determining anticompetitive effect due to mergers. In order to calculate market share of the company, it is necessary to specify the 'line of commerce' that is subject to calculation of the company's market share, and this is known as 'relevant market'. Defining relevant market for goods where competition is being questioned, is a prerequisite to determining anticompetitive effect of mergers with respect to the Fair Trade Act. However, defining the relevant market is more than just a precondition in determining anticompetitive effect via mergers; it has a critical role in determining whether the merger restrict competition. Relevant markets refer to the scope of enterprisers which are co-competitors, and thus, broad definition of relevant market lowers the market share or market power of the company thereby alleviating the evaluation standard of anticompetitive effect via mergers. On the other hand, if relevant market is defined narrowly, the opposite effect results. This shows that in determining the illegality of mergers, reasonable and accurate definition of the relevant market is imperative. Therefore, in many merger review cases, the recipient assert the market in mergers defines broadly, which brings the issue of ddefinition of relevant markets an essential issue in merger review cases. This paper thus analyzes how relevant markets defines in Free Trade Commission's decisions related to review of mergers for future in-depth studies on relevant markets.