The Application of the Lex Favor Reo Principle in Indonesia’s National Criminal Code Transition: Legal Construction and Legal Certainty

Abstract

The enactment of Law Number 1 of 2023 concerning the Criminal Code (KUHP) on January 2, 2026 marks a transition in Indonesia’s criminal law and raises issues concerning cases committed before its entry into force but examined or adjudicated thereafter. This study examines the legal construction of the lex favor reo principle under Article 3 paragraph (1) and Article 618 of the National Criminal Code and analyzes the implications of Law Number 1 of 2026 concerning Criminal Adjustment for legal certainty during the transition period. This study employs normative legal research using a statutory approach and a conceptual approach. The results show that although the National Criminal Code recognises lex favor reo as a mechanism for applying more favourable criminal-law provisions, Articles 3 and 618 do not provide clear parameters for determining the degree of favourability between competing legal regimes. The problem becomes more complex under the double-track sanctioning system, in which criminal sanctions and measures cannot be compared solely on the basis of imprisonment. Furthermore, the simultaneous implementation of the Criminal Adjustment Law introduces a further temporal layer that creates a moving norm condition and may generate uncertainty regarding the legal benchmark for applying lex favor reo. This situation contributes to divergent interpretations and unequal treatment in transitional cases. The study recommends that the Supreme Court establish technical guidelines providing a methodology for comparing criminal-law regimes and determining the law most favourable to the defendant during the transition period.