<正>A three-tier hierarchy consisting of a principal, an auditor and a manager is examined. Since the auditor can be honest or dishonest and the manager has the trend of collusion with the auditor, the principal must decide whether to allow or deter collusion. Deterring collusion is costly because the principal must reward the auditor to prevent him from accepting a bribe from the manager. Allowing collusion is also costly because the dishonest auditors will erode the deterrence of the punishment. An optimal scope of allowing or deterring collusion is put forward. Through analyzing and reasoning we show that if the auditor is honest with high probability, the principal should allow dishonest auditors to collude with the manager. As long as the reliability that the auditor offers information is greater than certain number value, hiring an auditor is useful to the principal. When the punishment pm is high enough, allowing collusion always turns out to be optimal.