M & A indeed is one of speedy growing methods for enterprise. But not every one enterprise can get its effectiveness as expectation after conducting merger. There is usually a huge organizational change is hidden behind M & A. The retained employees after the merger face the new company, seems like new employees that just inaugurated, filled with strange and a sense of uncertainty. In 1998, Watson Wyatt Company made a survey and found most of the merger failed to achieve the strategic target as expectation, M & A will ultimately lead to failure or poor performance, in addition to cultural differences, "people" issues can not be integrated effectively by the system is a major key factors, such as the leave of excellent talents, staff remaining in the low efficiency, So triggered the human resources issues discussed after M & A. The purpose of this study was to research successful key factor of human resources integration in the process of M & A which through an interview with qualitative, considering the time appropriateness and the difficulties of backtracking information validity to avoid making poor effectiveness. Especially to choose a case of retailer Taiwan in 2006 as example, French Carrefour merged with British Tesco to study. Also considering the impact of mergers and acquisitions on the acquired employees are greater than the acquiring ones, especially the employees under the middle management level, we selected the remained employees under the middle management level in the acquired corporation as the sample. This empirical study found that the unmet expectation of acquired employees not only from job regulations and benefit, but also from the person-organization fit of the acquiring corporation. The unmet expectation may both affect the psychological and behavioral stress of acquired employees. It also affects the organizational commitment of acquired employees to the acquiring corporation. The unmet expectation has a direct effect on the job stress and organization commitment. It also has an indirect effect on the performance positively through the perceived organizational support. The purpose of this study is to explore the human resources integration challenges under merger and acquisition of the pharmaceutical company A. The findings are from focused interview with employees of different backgrounds including acquired company employees, acquiring (A) company employees and top management team of the A company. The research results indicate that¡G 1.Under M&A, the internal integration team with outsider consulting firm¡¦s professional assistance will minimize the resistances. 2.Under M&A, the specific HR integration structure is needed. 3.Under M&A, the comprehensive communication plan is needed to erase the rumors and gossips. 4.Under M&A, the talent deployment must retain all the talents from both-side companies and eliminate the poor performers in the acquiring company. 5.Under M&A, for ensuring every colleague in the new company move forward to the same goals, the enterprise has to review the performance appraisal and management system.