In Taiwan, the Principle of Disadvantage Prohibition is applied not only to the civil or criminal procedure but to the administrative remedy procedure. Therefore, this principle, meaning that the agency with jurisdiction of administrative appeal shall not make less favorable modification or decision for the appellant than that was sustained in the scope of the objection which the administrative appellant expressed, also functions under administrative appeal related to taxation affairs.Without provisions in the Administrative Appeal Act, this prohibition, which is derived from the Judicial Precedents and interpretations in Taiwan’s early legal practice, is regarded as a jurisprudence of administrative remedy and applies to not only administrative appeal related to taxation affairs but recheck in the Tax Collection Act. This prohibition still has caused fierce controversy among the administrative law academic field in Taiwan ever since article 81 was established in the Administrative Appeal Act in 1998. On the basis of the principle of taxation by law, studies based on German Tax law criticize that article 81 brings tax unfairness and hold that the prohibition is neither a jurisprudence of administrative remedy nor a legal principle of administration remedy and therefore unjustified. Due to the fierce controversy, it is highly necessary to inspect the basis of legal theory of this prohibition.This study, within the scope of the administrative appeal related to taxation affairs, firstly intends to explore the meaning of this prohibition, the judicial decisions as well as the differences between Taiwan and Japan legal practice and theory. After observing and organizing the opinions for and against this prohibition, this study examines the legitimacy of this prohibition, and then compares the provisions in Taiwan and Japan whereby either the Tax Administration correct deficiencies or a Taxpayer may come forward voluntarily to a Tax Officer to correct past tax filings and deficiencies with the international trend towards establishing taxpayer rights.This study concludes that the legal basis of this prohibition differs from the main purpose of the administrative appeal related to taxation affairs. Through an analysis of the Japan experiences and the reflection on the judicial decisions, the main purpose of the administrative appeal related to taxation affairs falling into protecting the rights and interest of the people justifies this prohibition. In Taiwan, it represents the significance of protecting the rights and interest of the people under administrative appeal related to taxation affairs that the judicial decisions comprehend the legal concept Klagebefugnis mainly protecting the rights and interest of the people, instead of ensuring that all administrative acts are carried out based on the principle of administration by law. Hence the prohibition in Taiwan is legitimate from the perspective of the main purpose of the administrative appeal related to taxation affairs falling into protecting the rights and interest of the people. Japan’s legal practice and judicial decisions, which result from the main purpose of the administrative appeal related to taxation affairs falling into protecting the rights and interest of the people, may also help us to propose some suggestions on the reform and amendment of the taxation system in Taiwanese legal system. Howerer, all administrative acts, including collection of taxes, should be carried out in pursuance of a fair, open and democratic process based on the principle of administration by law. In Japan, the tax deficiencies get corrected after the administrative appeal, which this prohibition is applied to. With the international trend towards establishing taxpayer rights, the articles of Model Taxpayer Charter developed by three tax professional bodies in the the world, the Conf?d?ration Fiscale Europ?enne (CFE), the Asia-Oceania Tax Consultants? Association (AOTCA) as well as the Society of Trust and Estate Practitioners (STEP), and the provisions of taxation laws in Japan may help us to propose some suggestions on the reform and amendment of the taxation system in Taiwanese legal system.