Institutional investors can either monitor for shared gain or simply trade for private gain. If they choose to monitor during their investing, institutional ownership positively correlates with the firm value. Particularly, institutional monitoring can bring about a significant difference in the process of mergers and acquisitions(M&A). From the perspective of institutional monitoring, this paper examines the relationship between institutional investors and acquirers' performance of M&A's. And it examines whether institutions interact with majors in the acquirers' performance. The data covers 69 firms listed on the Korean securities market, which completed M&A from 2000 through 2004. The results of the analyses can be summed up as follows: First, institutional investors affect acquirers' performance of M&A's. The ownership of institutions are positively correlated with announcement return(CAR(-3,+1)) as well as long-run performance(BHAR(1-6month)) of M&A's. Second, the more major stockholders own equities, the more positively correlated the institutions are with announcement return(CAR(-3,+1)).