The perpetrator's belief in the victim's forfeiture of life (mahdour al-dam) represents one of the most challenging and contentious issues in Iranian criminal law, addressed under Article 303 of the Islamic Penal Code (adopted in 2013). This provision stipulates that in cases where the perpetrator claims to have committed the offense under the belief that the victim's blood was forfeited, the burden of proving such a belief rests upon the perpetrator, and obliges the court to adjudicate upon it. Nevertheless, judicial practice lacks a uniform and coherent approach regarding the modalities of proving such a belief, its scope of application, the allocation of the burden of proof, and the role of the prosecutorial authority (public prosecutor's office) in this process. The present article aims to elucidate the evidentiary implications of the belief in the victim's forfeiture of life and to analyze the existing judicial practice in this domain. The central research question is whether the proof of such a belief rests exclusively upon the perpetrator, or whether the court may also ascertain it by considering circumstantial evidence and the surrounding circumstances of the case. Furthermore, it examines whether this belief must necessarily correspond to the actual forfeiture of the victim's life, or whether the mere establishment of a sincere and reasonable belief on the part of the perpetrator suffices to preclude the application of qisas (retaliatory punishment). The hypothesis advanced in this article is that judicial practice has gradually moved away from requiring strict proof by the perpetrator and has instead deemed it sufficient for the court to ascertain the belief based on reasonable presumptions and circumstantial evidence. The research methodology employed is descriptive-analytical, based on the study of judicial decisions, including those rendered by the Supreme Court of Iran. The findings indicate that, although the text of Article 303 places the burden of proof upon the perpetrator, judicial practice—by endorsing the evidentiary instrumentality of proof—emphasizes the ascertainment of the belief by any possible means (including by the court itself). Additionally, such a belief must be founded on certainty and possess a reasonable basis; mere assertion or subjective speculation is insufficient. In sum, the ascertainment of the belief in the victim's forfeiture of life is a fact-intensive process grounded in the circumstances of each case, necessitating judicial precision and adherence to both procedural and substantive legal standards.