本文探討我國所得稅制改革、盈餘管理及公司治理對有效稅率之影響；研究結果顯示，2010 年營所稅率由25% 下降至17%，因直接受惠於營利事業，大幅減輕企業租稅負擔，降低其有效稅率。再者，本文以裁決性應計數代理盈餘管理行為，研究結果顯示裁決性應計數愈大時，以降低租稅負擔為目的之盈餘管理行為愈大，將使得公司有效稅率下降。最後，本文探討良好的公司治理機制是否能有效減少管理階層不當盈餘管理行為，進而增加企業有效稅率；實證結果發現，機構投資人持股率愈高時，因實際參與企業營運的人愈多且專業程度愈高，愈能抑制管理階層之盈餘管理行為，降低不當租稅規劃，進而提高企業有效稅率。
This study investigates the relationship among income tax system reform, earnings management, and corporate governance on the effective tax rate. Taiwan's government has amended its income tax law and reduced the income tax rate to 17% in 2010, in which the income tax is deflating the effective tax rate. This study further used discretionary accruals to proxy for earnings management behavior, showing that management of a business with more discretionary accruals targets reducing the tax burden through earnings management behavior, with greater earnings management implying a decline in the effective tax rate. Finally, we looked at whether good corporate governance mechanisms can effectively reduce the improper tax planning of earnings manipulation and increase the effective tax rate. We found that the characteristics that help better inhibit management's earnings management behavior, reduce inappropriate tax planning, and increase the effective tax rate include a higher institutional investors holding ratio with more discretionary accruals, as well as more people who understand the actual operations and have more professional knowledge.