According to the views expressed by the Judicial Yuan, the term “law” under the principle of taxation by law refers to the statutory laws enacted by the Legislative Yuan as well as the regulations prom-ulgated by the administrative agencies under statutory authorization; however, such views are inconsistent to the fundamental value under the theory of significance in light of the international legislative trend and Taiwan’s long practice of constitutional interpretations. Therefore, it is necessary to look for a proper opportunity to shift to the principle of strict congressional reservation. The principle of taxation by law involves people’s fundamental rights, such as people’s property rights, right to work, right to live, and right of freedom in general and peo-ple’s right of free movement and right to institute administrative ap-peals and right to institute legal proceedings under individual tax laws in specific. The application of the principle of taxation by law requires the utilization of various methods of interpretation. Among all, the substance-over-form doctrine belongs to the interpretation based on legislative purpose but in concept it shall be distinguished from the principle of taxation based on facts. Under the principle of taxation by law, how to distinguish the regulation of tax bases and regulation of ascertaining facts is widely controversial in constitutional interpreta-tion practices. The former shall cover the scope of the subject and ob-ject of a specified taxpayer and the method of calculating amounts of tax. The latter shall cover what evidence rule shall be applied to ascer-tain facts and what facts shall be ascertained when a taxpayer fail to provide evidence in violation her cooperative duty.