This case discusses the history and competition of Korean mobile communication industry, and the marketing strategy that SK Telecom executed during the period of 3rd generation technology transition. At the beginning of the year 2007, KTF, the number two player in Korean mobile industry, aggressively opened its 3rd generation mobile communication service before SK Telecom. The situation was regarded as a threat to the current leader, SK Telecom. The counter strategy for SK Telecom to maintain its market leadership will be discussed. Also, subjects that should be considered at time of technology transition, such as new brand strategy, relationship with competitors in early market environment, and killer application, will be examined.