The International Crimes Tribunal Bangladesh (ICTB) was set up by Bangladesh through the adaptation of the International Crimes Tribunal Act 1973, as an internal mechanism trying to prosecute and punish Bangladeshi perpetrators who committed international crimes in Bangladesh liberation war in 1971. After a long disappearance from the public eye, the Tribunal was reemerged in 2010. The recent cases decided by the Tribunal have revealed that the international crimes; namely, crimes against humanity, were allegedly committed in 1971, while the relevant Statute was enacted in 1973, and was implemented in 2010. Recently, the ICTB is prosecuting crimes against humanity retroactively, which might have violated the prohibition of penalizing certain conducts committed by the perpetrators before the enforcement of such conduct as a law banning such demeanor as an offense. Therefore, this study firstly analyzes the rule against retroactivity in international criminal law. Secondly, it investigates the justification of the retroactive criminalization of crimes against humanity at the first International Military Tribunal, Nuremberg, and its crystallization into the regional and international legal instruments. Thirdly, the study examines the characteristics of crimes against humanity as an international crime, to scrutinize whether the ICTB needs to fulfill such requirements either in 1971 or 2010. Then, it illustrates various judgments of the ICTB, demonstrating that it does not comply with the rule prescribed by international laws either in 1971 or 2010, in prosecuting crimes against humanity retroactively. Lastly, the study concludes by forwarding ways necessary to the ICTB in retroactive prosecution of international offenses.