The purpose of this study is analyzing the effect of ownership structure on the relationship between related party transactions and investment efficiency. In this study, various forms of ownership structure were used; they include managerial ownership, institutional ownership, its manner of influencing the relationship between the related party transactions and investment efficiency among the 65 companies listed in Tehran Stock Exchanges in a ten-year period from 2005 to 2014. The results of study showed a negative and meaningful relationship between the related party’s transactions and investment efficiency. They, as well, implied that the ownership structure (managerial ownership and institutional ownership) has a positive effect on the relationship between the related party transactions and investment efficiency. The general outcome of the study showed that the related party transactions have been opportunistic, and applying appropriate monitoring mechanisms or using them, at least, in case of the related party transactions can minimize the conflict of interests and the opportunistic manners of the managers.