This study relies on a customer demand-based view to examine how mobile number portability affects competition in the Korean mobile telecommunications market. Using a contingent valuation method, we investigate 1,161 subscribers' willingness to pay for mobile number portability The findings show that there is a difference in demand for number portability among the subscribers to service providers. Unlike previous studies on number portability, the introduction of number portability to the market in which the brand effect of a dominant player exists would have an adverse effect on competition in the mobile telecommunications market. The study, also shows that if the market structure is asymmetric with a strong, dominant player, some additional regulatory mechanisms are needed for the facilitation and the implementation of number portability in order to reduce the side effect of it. Implications and avenues for future research are discussed in the conclusion section of the article.