The principle of legitimate expectation (Vertrauenschutzprinzip) concerns the protection of the people's rights and interests under the Constitution of a state. The main purpose of this article is to introduce the three elements of the principle of legitimate expectation (including the basis of trust, the displaying of trust and the need for trust to be something worth protecting). Then, based on the principle of legitimate expectation, the paper discusses Taiwan's tax policy for teachers. Finally, it reaches the conclusion that teachers should be taxed. Yet, inasmuch as the expectation that teachers must pay taxes abridges the interests of teachers who had a legitimate expectation of enjoying these interests, the government should provide a reasonable plan for reimbursement with a view to minimizing the loss to teachers' rights and interests. In this way it will comply with the Taiwanese Constitution's objective of protecting the people's rights and interests.