This study aims to evaluate the principle of proportionality as a universal legal conformity criterion, specifically within the context of tax crimes and penalties in the Turkish legal system. First, a literature review has been made about the principle. Following the literature review, the tax penalties which were subject to appeal by the Supreme Court and to norm control by the Constitutional Court, have been examined in detail based on all the aspects of the principle of proportionality. Eventually it was concluded that the principle of proportionality has an efficient role in determining tax penalties' compliance with the law. Even though the principle of proportionality has been qualified as a constitutional norm in the Turkish legal system, it has been revealed that the principle was included implicitly at the level of the law based on the Articles 3 and 61 of the Turkish Penal Code and that the Supreme Court has been making its auditing decisions regarding the principle based on the relevant articles.