本論文探討兩稅合一制所增設的兩個機制，即稅額扣抵比率及加徵保留盈餘稅對上市公司股利發放、資本結構以及公司租稅規劃的影響。首先以數學模式推導出兩稅合一制下這兩個機制與股利發放率及負債權益比率之間的關係，再據以建立研究假說，而後進行兩步驟多變量程序之實證測試。研究所獲致的結果包括：
　　一、兩稅合一制實施後，上市公司稅額扣抵比率與總（盈餘）股利發放率呈正相關，且上市公司稅額扣抵比率愈高者，其總（盈餘）股利發放率、現金股利發放率及盈餘配股發放率皆較兩稅合一制實施前高。由於公積配股並不受兩稅合一制實施的影響，故本文也發現兩稅合一制實施前後公積配股率並無顯著差異。整體而言，兩稅合一制實施後，總（盈餘）股利發放率、盈餘配股發放率及公積配股率較實施前減少，而現金股利發放率較實施前增加。進一步的分析顯示現金股利發放率的增加係受到健全股利政策的推行影響，而盈餘配股發放率的減少可能是受到股票股利緩課取消的影響。
　　二、兩稅合一制實施後，符合促進產業升級條例第八條及第九條規定之投資方案，選擇股東投資抵減之公司並無顯著多於選擇五年免稅之公司，可能原因為選擇五年免稅有利於公司現金流量及每股盈餘的金額。此一結果雖與預期方向相反，但也顯示政府原先擔心公司會選擇股東投資抵減而造成鉅額稅收流失的疑慮並未獲得實證支持。但從兩稅合一實施後公司股權結構的變動分析顯示，公司法人持股比率較實施前增加，而僑外投資持股率較實施前減少，支持公司在不同股東身分稅負差距的考量下，會有股權結構移轉之租稅規劃，以降低股東所負擔的稅負。
　　三、兩稅合一制實施後，上市公司稅額扣抵比率愈高者，股東所享扣抵權愈多，則舉債租稅利益愈少，因而總負債權益比率將降低；而上市公司保留盈餘比率愈高者，所課徵的保留盈餘稅愈高，舉債利息的租稅利益反而增加，會造成總負債權益比率增加。本文的實證結果也顯示，兩稅合一制實施後，上市公司稅額扣抵比率愈高者，其總負債權益比率較實施前低；而上市公司保留盈餘比率愈高者，其總負債權益比率較實施前高。
　　綜合本論文實證結果顯示，兩稅合一制實施後，稅額扣抵比率的機制確實其有消除股利所得重複課稅的效益，進而提高公司股利發放的意願，並且有助於公司資本結構的改善。但是，政府對於未分配盈餘加徵 10％營利事業所得稅的規定卻不利於公司資本的累積，反而提高舉債的租稅誘因，也可能抵銷稅額扣抵比率降低舉債的效果，值得政府深入檢討對公司未分配盈餘課稅的不利影響。

　　Enacted in 1998, the Integrated Income Tax System has two important features, i.e., the imputation tax credit to shareholders and the 10% surtax levied on undistributed earnings. Using two-step multivariate regression, this study exmines the impacts of these two features on corporations' dividend payout, tax planning, and capital structure. The findings of this study are as follows :
　　1. After implementation of the Integrated Income Tax System, the imputation tax credit ratio is positively related with total dividends payout ratios. Companies with higher imputation tax credit ratios tend to pay out more cash dividends and earnings stock dividends, ceteris paribus. However, there appears no significant relation between the imputation tax credit ratio and capital stock dividends. On average, after implementation of the Integrated Income Tax System, stock dividend payout ratios, including earning stock dividends and capital stock dividends, are decreased, but cash dividend payout ratios are increased. The increase in cash dividend payout may be affected by the balance dividend policy by the SEC, which require corporations to pay out greater proportions in cash dividends. The decrease in earning stock dividends may be due to the cancellation of the deferral of tax on stock dividends.
　　2. After implementation of the Integrated Income Tax System, inconsistent with the expectation, companies still prefer the five-year tax exemption to the investment tax credits. The results suggest that companies' choices are affected more by such factors as corporate cashflows and earnings per shares. However, after implementation of the Integrated Income Tax System, there is a significant change in stockholder structures, with the ratios of corporation stockholders being increased while the ratios of oversea stockholders being decreased. The opportunity of tax arbitrage will have a significant impact on corporations' stockholder structures.
　　3. After implementation of the Integrated Income Tax System, for companies with higher imputation tax credit ratios, their stockholders will have greater tax benefits, and, hence, the tax benefit for companies to use debt financing is decreased. Conversely, companies with higher ratios of retained earnings will bear more tax burdens on the 10% surtax levied on undistributed earnings; therefore, the tax benefit of debt financing is increased. The empirical results support that, ceteris paribus, companies with higher imputation tax credit ratios tend to have lower debt-to-equity ratios, while companies with higher ratios of retained earnings tend to have greater debt-to-equity ratios.
　　In summary, after implementation of the Integrated Income Tax System, the imputation tax credits reduce the double taxation on stockholders' dividend income and, hence, improve corporations' capital structure. However, the 10% surtax on undistributed earnings may result in opposite effect on corporations' capital structure. The findings of this study have important implications for the government to take into account the potential adverse impacts resulted from taxes levied on corporations' retained earnings.