Song and Walkling(2000)finds the rivals of acquired firms earn positive abnormal returns and develops “Acquisition Probability Hypothesis” as a explanation. This paper examines the effect of takeover rumors on the stock price of the target firms and rival firms, using market model and three-factor model to test if target firms will earn abnormal returns and the rivals of target firms will earn positive or negative returns when takeover rumors exist? Meanwhile, this paper also examines if clarification will have the negative effect on the stock price of acquiring firms and target firms, and furthermore tries to find what factor will affect the cumulative abnormal returns of target firms and rival firms.