Daum Communications Co. was established in Eeb. 1995. Daum was growing with providing first free e-mail service in Korea, listed on the KOSDAQ, relieved from deficit and generated actual profit in the 2002. At present, Daum has most bigger Internet business company both quality and quantity with 5,500million membership, 2,000 employee and total sales 500billion(￦) in the 2005. These history decade of Daum is challenge and counter with change and evolution of Korea Internet business environment. Daum faced competition is, however, not favorable circumstances. Daum has to overcome and prepare competition for the 1st internet business firm with NHN with knowledge search service, rising competitor telecommunication company with convergence service, and any other potential rivals. Daum restructured its service and organizational structure in order to appropriately response for expected change of Internet business circumstance in the Aug. 2005. Providing service was rearrangement 4 core division as “Daum Media”, “Daum Shopping”, “Daum Finance”, “Daum Global” and structure was reorganizationed the direction of maximizing efficiency and effectiveness and creating mutual synergy, This case investigated Daum’s strategy of “selection and concentration” for acquiring competitive advantage in the past ten years with changing Internet business surroundings