The objective of this research is to understand the long term impact of the internal as well as external merger and acquisition toward shareholder prosperity using size and beta adjust¬ment abnormal return which had already been shown by the study of Dimson & Marsh (1983). This research based on sample of companies that listed on Jakarta Stock Exchange and perform merger and acquisition during 1999-2000. The results show that there is no significant in¬fluence of merger and acquisition toward shareholder prosperity. There isn't any significant differ¬ent of AAR and CAAR of companies which perform internal as well as external merger and acqui¬sition. Furthermore, the AAR and CAAR of companies before and after merger and acquisition are insignificantly different.