The article deals with the problematic issues of substituting the life imprisonment punishment with more lenient one. It is stated that before being recently amended, the Criminal Code of Ukraine did not foresee the possibility of such substitution, so that the life convicts could be exempted from the further serving the punishment because of the illness or being pardoned. According to new Part 5 of Article 82 of the CC of Ukraine, a punishment of life imprisonment may be substituted by a punishment of imprisonment for a term of fifteen to twenty years, if the convict has served at least fifteen years of the punishment imposed by the court. If one analyzes this provision from the point of view of the arguments set forth in the decision of the European Court of Human Rights in the case "Petukhov v. Ukraine (No. 2)" and draw parallels between the procedures of pardoning and substituting the punishment of life imprisonment with more lenient one, then a similar question will arise: are at least fifteen years of the sentence imposed by the court and served by the convict included in the term of the punishment in the form of imprisonment, assigned in the order of substitution for a term of fifteen to twenty years? The article presents a number of arguments in favor of a negative answer to this question. In particular, it is stated that the term during which the punishment in the form of life imprisonment was actually served is taken into account as one continuous term, and is taken into account only once as a condition for substituting the punishment. As well, the author comes to the conclusion about the impossibility of partial counting of the term during which the person served the punishment of life imprisonment (that is, in the part that exceeds the established in Part 5 of Article 82 of the CC of Ukraine minimum required for the substitution the fifteen-year term) into the term of imprisonment appointed in the order of substitution. Firstly, the criminal law does not establish the criteria for such splitting of terms, and secondly, then the procedure of life imprisonment substituting can create significant prerequisites for abuses during its implementation. The court cannot retrospectively assess which term of actual serving the punishment is sufficient, but shall assess it at the very moment of considering the motion for substituting the punishment.