傳統最適所得稅文獻關切的議題主要在政府與納稅人的訊息不完全衍生的租稅問題，而忽略了勞動市場的訊息不完全對租稅設計造成的影響。這個結果導致在傳統的架構下，享有訊息租 (information rent) 的是高能力者而非低能力者，因而傳統租稅設計的目的都是讓訊息真實的顯露以提高低能力者的福祉。然而，我們認為傳統文獻忽略了勞動市場存在逆選擇問題的可能性。當勞動市場存在逆選擇時，低能力者將有誘因偽裝為高能力者來獲取較高的薪資而享有訊息租，這個性質將使得廠商基於效率目標所採行的篩選機制與政府所得重分配的目標背道而馳。因而在本文的設定之下，最適的所得稅制應該讓高能力者的邊際稅率等於1，亦就是說，政府應該透過租稅的設計來破壞廠商的篩選機制來達到所得重分配的目標。 This project explores government’s income tax system under information asymmetries and uncertainty of labor market. When the adverse selection problem exists, firms cannot observe worker’s ability directly. However, an employee’s output depends not only on his ability but also on uncertainty in the economy. Thus an efficient labor contract with pay-for-performance will separate good workers from bad ones. In this circumstance, we consider how government can implement redistributive policy with income tax system. In contrast to traditional models of Mirrlees(1971) and Stiglitz(1982), which show that the marginal tax rate of high ability workers should be zero because of their information rent, our model shows that the low ability workers have incentives to mimic high ability workers and own information rent. According to the contract theories, we prove that an efficient contract will distort the high ability worker’s decision. Therefore, the basic characteristics of optimal income taxation are different to traditional ones. Marginal tax rate is no longer zero for high ability workers. On the other hand, by ignoring worker’s incentives, we get a pooling solution, which shows that a pooling contract equilibrium is optimal.