Constitutionalization is without doubt one of the most significant changes in Finnish criminal law. Traditionally criminal law and constitutional law have been separated in legal science and in practice. There are，however，certain features and developments that inevitably move them closer to each other and even connect them in a manner that does not respect the traditional boundaries of these legal domains. The constitutionalization of criminal law is not uniquely a Finnish phenomenon but has also a wider scope that is related，inter alia，to the Europeanization of criminal law. The purpose of this article is to explore the constitutionalization of Finnish criminal law that has mainly taken place after the reform of the fundamental rights provisions of the Finnish Constitution in the mid-1990s. The purpose of the article is，thus，to explore the meaning and the scope of the rule of law within criminal law with first providing a brief overview on the general meaning of the rule of law. Secondly，the general trend or development of constitutionalization of Finnish criminal law is examined. Thirdly，perhaps the most significant and universal manifestation of rule of law within criminal law，principle of legality，is discussed.