本文探討在舊制與新制稅法，分別以課稅所得與財務會計所得爲課徵基礎下，影響我國上市櫃公司未分配盈餘加徵稅負之因素。本文的實證結果顯示，新制稅法下，繳納當期營利事業所得稅較低且保留盈餘比率高之公司，將會繳納較多因未分配盈餘予股東而加徵之稅負。但本文也發現，企業研發及固定資產之資本支出與未分配盈餘加徵稅負間呈現顯著之正向關係，顯示對未分配盈餘加徵稅負可能增加企業籌措資本投資資金的稅負成本。最後，公司規模、盈餘波動性、股東權益報酬率、流動性及負債比率等企業特性與未分配盈餘稅負間有顯著之相關性，顯示企業特性不同會影響其未分配盈餘加徵稅負之成本。 This study empirically investigates the factors influencing firms' surtax liabilities on undistributed earnings before and after changes in the tax basis of undistributed earnings. The tax basis of the 10% surtax on undistributed earnings was changed from taxable income (the old tax system) to book income (the new tax system) in 2006. The results of this study show that by adopting financial income as the tax basis, the new tax system can alleviate the distortions in firms' surtax on undistributed earnings caused by the book-tax income differences. The empirical results, however, indicate that there is a positive relation between firms' investments in R&D and fixed assets and the surtax on undistributed earnings, suggesting that the policy of taxing undistributed earnings may cause firms to bear higher tax costs on investments. Finally, the paper documents the evidence that firms' surtax on undistributed earnings is associated with several firm characteristics such as firm size, volatility of earnings, returns on equity, liquidity, and debt ratio.