State decree is one of the main instruments in the administration of the Islamic society in the hand of the religious ruler through which the system will have a special power to resolve the social issues of the Muslims. This power will be possible if the expedience is provided. Expedience can mean something against the corruption or the opposite of corruption. As two main fiqhi schools of Imamiyya, schools of Najaf and Qom have different approaches about the function of state decree in repelling corruption or attracting expedience due to different bases and ijtihadi methods. Considering as a criterion the protection of system in the establishment of the state, the school of Najaf believes that the function of state decree must be in repelling the corruption from the system, therefore, state decree will be permitted to be issued if secondary subjects are realized. Due to the belief in absolute Wilayat Faqih, the school of Qom considers the function of state decree in providing public welfare and desirable expedience in addition to repelling corruption which strengthens the state decree to make the Islamic system more efficient.