本研究旨在探討我國2009及2010年公告營利事業所得稅率由25%調降為17%，企業於財務報表中認列「所得稅率變動影響數」之價值攸關性。本研究是探討此次因修法所造成的所得稅率變動影響程度，是否會對於當期的盈餘產生重大的影響，且當投資人面對其盈餘時，是否會分辨此影響盈餘的暫時性因素，且是否會對企業作理性評價。研究結果顯示，我國營利事業所得稅率的大幅調整，對於有淨遞延所得稅資產與淨遞延所得稅負債企業，其相對認列的所得稅率變動調整數，確實占法令變動年度的盈餘極高的比重，對此類型企業之盈餘報導會產生重大的影響效果。其次，所得稅率變動調整數之價值攸關性則是與其他所得稅費用內含相近，符合配合原則費用之概念。雖然所得稅率變動影響數是暫時性項目，但投資者對此項目之評價效果高於對其他所得稅費用之綜合淨額及損益表中暫時性項目的評價效果。本研究根據淨遞延所得稅資產、負債分組，測試其股價累積超額報酬是否存在顯著差異。在法令公布時，兩組之累積超額報酬並無顯著差異，而是等到年度財務報導發布後一季間會存在重大影響。但因為其是暫時性項目，此效果在數月後即不復見，此合乎功能性固著現象，投資者不會區分盈餘之內含，未將此項變動調整數放入其盈餘預測中，造成短期錯誤的評價現象。

This study examines the valuation of the tax effects which was induced by the income tax rate was reduced from 25% to 17% during 2009 and 2010 in Taiwan. It's good news for firms who will reduce tax burden and increase profit, but the results were shock for those firms who had large deferred tax assets. These firms had negative effects on the earnings of the year when the corporate tax rate reduced, the reason for the sake of deferred income tax. Therefore, this study examines the short-term changes in earnings when the result of amending the law, have or not a significant impact on current period earnings, then investors whether distinguish changes of the earnings is temporary and no persistent, and whether make a rational pricing for the stock. The results showed the tax effects were very great proportion of earnings. The regression analysis showed that tax effects for investors, similar other tax expense components whose attribute as expense. The results show that the investors are functional fixation of the reporting numbers, and are reflected in the share price which could prove that investors do not understand the temporary property of the tax effects and don't make a rational pricing for the stock in short term.