AT&T, a major voice and data communications company of the U.S.A., had carried out three restructuring programs since the 1009s: the restructuring of the main body into three separate companies in 1995, the aggressive diversification strategy during 1997-2000, and the restructuring plan in October 2000.Since October 1997, new designated CEO Michael Armstrong has propelled aggressive diversification strategy that is based on telecommunication service. His main objective is to make AT&T group a service bundling firm that provides data, voice, cable TV, wireless, and internet services simultaneously in order to acquire the economies of scope and the first mover’s advantage in newly growing markets. AT&T has expanded its activities to various business domains through M&A, joint venture, or strategic alliances.However, the performance derived from diversification strategy was below expectation. The low performance and excessive debt that was used to enlarge business activities made AT&T’S stock price fell down. In October 2000, AT&T announced restructuring plan to create a family of four businesses. Under the plan, which was expected to be completed in 2002, each of these four businesses will become publicly held and traded as either a common stock or tracking stock.After analyzing the causes that AT&T has changed the strategies from service bundling to service focusing withing three years, we find that changes in market environment and stakeholder’s needs influenced AT&T to establish its business strategy.