The subsidiary religious laws, in the sacred Islamic law, are extracted and inferred using the book (Quran), tradition, consensus (among Fokaha) and wisdom. However, it is very general in nature and no more specific details had been set from the beginning. The great mujtahids and scholars of Fiqh embarked upon the problem in order to solve it. Accordingly, Istishab is considered as one of the oft-used principles in this respect. It is where a situation existing previously is presumed to be continuing at present, although there is some doubt, it presumes the continuation of the fact until the contrary is proven. For example: we know that someone was certainly alive in 1971, and was missed in 1991 when his father died. Doubt arises as to whether a man is still alive and can inherit his father's wealth? The previous certainty, in this case, is presumed to be continuing at present that he is known to have been alive. In case of doubt, on the contrary, it is assumed that he is deceased. However, by istishab, the presumption will be that the death is not proven. Because the principle of Istishab, has a privileged position in civil law and Imamieh jurisprudence. The purpose of this study was to analyze the role of Istishab in civil law and Imamieh jurisprudence.