Compare with commercial advertises on mass media, product rumors could be counted on as the negative advertises among the consumers. The characteristics of allowing anonymous users of BBS (Bulletin Broad System) on Internet make an environment that rumor, not confirm message, appearing once and once again. The communication within BBS is similar with person-to-person oral communication in some way. It makes the rumors be believed easy. Not only having person-to-person communication characteristic, the communication within BBS has the ability in diffusing the messages to the unspecified public. This makes BBS be a media full of rumors. A real case of rumor had been used as an example to discuss the source credibility and the consumers' attitude to the content of the rumor. Longitudinal data collect were adopt for the study. Two questionnaires survey were managed on just one month after the rumor appearing and a half of year later to know the consumers' attitude to the rumors. After the empirical study, we find that product rumor would influence the consumers' attitude as we expected. Although having question on whether the rumor was true or not, some subjects on the study still changed their attitude on the product. It's worth for the firms to keep sight on this. It was also found that the consumers' attitude to the rumor is depended largely on whether the rumors sound reasonable. Refuting rumors by objective professional persons had significant effect, but the clarity made by the firm didn't work well enough. It's difficult to do empirical research about rumors. But it's an important issue needed to explore, especially for rumors on BBS. From the results of this empirical study, we found that people would change the behavior even if the do not believe the rumor. Managers should remember this when they have to face the rumors that may hurt their companies.