When applying competition laws, we need to first analyze whether atelecommunications provider possesses sufficient market power to allow it to dobusinesses in an noncompetitive manner. It is also necessary to determine whichmarket is being dominated. What should be the boundary? How many markets shouldbe included? All these are reasonable questions for market analysis. Such questionsare referred to as “market definition” to arrive at a “relevant market” outcome. In brief,it is sufficed to say that the larger the relevant market, the lower the market power, orvice versa. Market definition is then decisive to the whole process of market analysis.However, it does not follow that we first need a relevant market, in order to determinemarket power. This article is aimed at explaining how to theoretically and practicallydefine relevant market with examples from the telecommunications market that wouldbe conceptually useful for Thai audiences and Thai competition laws.