The portability of telephone numbers is one of the main regulatory issues in the Korean mobile telecommunications market. The availability of mobile number portability is likely to bring substantial benefits to subscribers such as lower price, greater choice, higher quality and a greater range of services. In this paper, we used willingness to pay (WTP) to elicit demand on number portability in the Korean mobile telecommunication market. In doing so, the study has three goals: i) to introduce the contingent valuation method to the telecommunication field and to obtain WTP estimates for mobile number portability so that we can examine how estimated WTP varies with socio-economic characteristics such as age, gender, income and education etc.; ii) to examine whether a so called "brand effect problem" exists or not in the Korean mobile telecommunication market, and if so, how it would affect competition in the Korean mobile telecommunication market; iii) to find out a way of facilitating the implementation of number portability in the Korean mobile telecommunication market.