Since related party transaction is confidentially made by insider agreement, the manager has possibility to use it in decision making with his own arbitrariness. Therefore, to explain if the manager make a decision with related party transaction which has this particular feature is highly important in the academic field. In this study, we consider that how related party transaction effect on the information asymmetry and wealth of investors through the cases of seasoned equity offerings; S.E.O which is a main business decision of firms. We specifically study that if they increase related party transaction before their S.E.O as an opportunistic behavior for their advantageous S.E.O and how this kind of related party transaction effects on their long term stock performance after the S.E.O. We also studied the structural relation of the causes through the three-step mediated regression analysis method when related party transaction influence to the long term stock performance. The result is as in below. First, the firms which do S.E.O increase related party transaction before the year they do and they are the firms which have high volatility of related party transaction. Secondly, the firms which have higher volatility of related party transaction report lower long term stock performance than the firms which have lower volatility of it. Thirdly, we verify that high volatility of related party transaction causes the increase of information asymmetry as it raises financial opaqueness and it makes depreciation of long term stock performance after S.E.O. These studies provide implication as follows. The existing studies regarding related party transaction have examined the earnings management from related party transaction and how related party transaction effect on management performance. However, in this studies, we examine that related party transaction not only effects on reporting earnings but also exposes the investors to the information risk. Moreover, the results explain that the information risk causes serious loss to the investors due to the sharp falling of the stock return after S.E.O. Therefore, related party transaction can be used as a tool of the manager’s opportunistic behavior, and the investors need to be careful about this. Through this study result, it is possible to investigate if related party transaction has the right function which improves business performance or it is just a method for the company’s private benefits. Moreover, it can fully support the social issue as a evidential data.