Ｒesearches on control mechanisms paid little attention to their dark side and conditional limits．Based on the transaction cost theory，social exchange theory and the contingency perspective，the paper explores the shifting effects of contracts，relational governance，as well as their interaction over the range of relationship duration and environmental uncertainty． The results reveal that both relationship duration and environmental uncertainty weaken the negative relationship between contracts and opportunism; relationship duration also weakens the negative relationship between relational governance and opportunism． Furthermore，contracts and relational governance function as substitutes at the early stage of channel relationships or in a relatively stable environment，yet they become complements in mature relationships or in highly uncertain environment．