單一稅率制度係對各類所得課以相同稅率的稅，具有簡化、效率與公平的特色。傳統的累進稅率制度，雖源起於量能課稅原則，但由於租稅減免與優惠政策的擴大，反而侵蝕稅基，破壞租稅的公平性並使稅制變得複雜。在維持91年度所得稅收不變的前提下，本研究的模擬分析發現營利事業及綜合所得稅之單一稅率可降為15%，約有43.5%的企業因低稅率而減少稅負，僅7%的企業由於租稅減免項目的減少而增加稅負。另當個人基本免稅額提高到220,000元時（身心障礙者另給予74,000元），35%的綜合所得稅申報戶，因免稅額的增加而減少稅負；36%的綜合所得稅申報戶則因捐贈扣除等項目的限制而增加稅負。且單一稅率每增加1%，政府稅收即增加約240億元，可用於長期最適單一稅率的模擬。本文建議於促進產業升級條例的租稅優惠項目將於民國98年停止適用之際，政府應研議實施單一稅率制度，以大幅減化現行稅制並促進租稅公平。

The flat tax system taxes all types of income with the same tax rate to achieve simplicity, efficiency, and fairness. Although the traditional progressive tax system is based on the ability-to-pay principle, the ever-expanding tax preferences, however, have eroded the tax base, compromised tax equity, and complicated the tax system. Under the assumption of preserving 2002 tax revenue, this study develops a tax simulation model to determine the impact of adopting a flat tax system in Taiwan. We find when a 15% flat rate is applied to both business and individual income, 43.5% of business income taxpayers will pay less tax, while only 7% will have more tax burden. As to individual income taxpayers, 35% are better off with less tax payments provided that personal exemption was increased to NT$220,000 and an additional special deduction of NT$74,000 for those who are disabled or handicapped. On the other hand, 36% of the individual income taxpayers are worse off with more tax exposure due to the elimination of deductions. Moreover, every 1% increase of the flat tax rate will increase total income tax revenue by NT$24 billion. This facilitates simulations of a long-term optimal flat rate. Our results suggest the government should evaluate the adoption of a flat tax system to simplify the tax system and to promote tax equity, before the tax incentives provided in the Statute for Upgrading Industries expire at the end of 2009.