This article is devoted to the problem of reforming Ukrainian legislation in accordance with the European principles of humanization and mitigation of criminal liability. Since the principle of supremacy of the law acts and is admitted in Ukraine, and the Constitution of Ukraine has supreme legal validity, laws and other normative legal acts have no retrospective effect, except those which mitigate or eliminate liability of a person accused of committing a crime. Amendments to the Criminal and Criminal Procedural Codes of Ukraine of 2008have proscribed that after a person has committed a сriminal act, and the Law on Criminal Liability was changed several times, the law eliminating the criminality of that act, mitigating criminal liability or improving a person’s position has a retrospective effect. In this regard, the Constitutional Court of Ukraine has to interpret some basic questions, such as: whether punishment in the form of life imprisonment can be applied to persons who have committed a criminal act for which until March, 29, 2000, the Criminal Code of Ukraine proscribed the possibility of the death penalty; and whether life imprisonment is a new kind of punishment or is a replacement of one kind of punishment with another one – in this case, the death penalty with life imprisonment. The author proves that the Criminal Code of Ukraine, with some modifications concerning mitigation of criminal liability, should be considered as a new criminal code. According to the legal position of the Constitutional Court of Ukraine, for persons who committed crimes, for which the possibility of the death penalty was proscribed until March 29, 2000, the punishment of the life imprisonment may be applied.