Remission is a legal instrument to relinquishor put a stop, at any time, on the implementation of any type of punishment for a convict after a verdict of a criminal court, of any type or degree, is legally finalized. Remission of a sentence or part of it may be awarded fully or partially, absolutely or conditionally, whether the sentence has been put into practice or not. This means that the law empowers an executive body such as the Head of State, a minister or the Cabinet-in the case of the Islamic Republic of Iran, the Supreme Leader on the request of the Head of the Judiciary Power- to relinquish a sentence being executed under its discretion within the law. The institution of remission has existed in Iranian penal law for a long period of time. At present, according to paragraph 11 of Article 110 of the Iranian Constitution and Section 24 of the Iranian penal law, the pardoning of a sentenced offender is granted by the Supreme Leader on the request of the Head of the Judiciary Power. Detailed regulations of how and when remission takes place and who may be a good candidate to be pardoned is determined in the By-law of Commission of Remission and Mitigation of punishment of Convicts 12/9/87 ratified by the Head of the Judiciary. Before the Islamic Revolution, the Cabinet was in charge of ratifying regulations, but after the Revolution, the Head of the Judiciary Power is in charge. The first By-law was ratified by the Cabinet in 28/7/1324. Later, the regulations were amended or completely changed and replaced by a new set of rules more than ten times, none of which took place so soon as did the By-laws of the Commission of Remission and Mitigation and Substitution of punishment in 16/5/87. This By-law did not live long and soon after its ratification, within 4 months in 12/9/87, it was replaced by a new By-law. The old By-law also introduced a new term substitution of punishment to be awarded under the regulations of remission. This replacement was very sudden and unusual, and no reason or formal explanation was provided for the event. More importantly a key word, namely substitution of punishment was omitted from the title of the new By-law but remained un-touched in the text. Accordingly, this essay is designed to discuss the changes, particularly substitution of punishment by means of remission, in the light of an analysis of the definition of remission in law and a historical account of its implementation in conjunction with a comparative analysis of the last two By-laws, i. e. the By-laws of 16/5/87 and 12/9/87. It is concluded that the new By-law (12/9/87) is ultra vires and is in sharp contrast with the law where it recommends substitution of punishment to be graced by means of remission. This is because the term substitution of punishment is not in harmony with the literal and legal definition/meaning of remission. Legally speaking, substitution of punishment is a task which has merely been bestowed on courts that can be decided upon when passing a sentence. Thus, an executive body, the Commission for Remission cannot forfeit this legal power. Moreover, this will harm the principle of independence of the courts and judges and will undermine stability of their verdicts.