Knowing the sources of every scientific discipline is a basic necessity in its scientific usage. In the field of the knowledge of rejal, there are tens valuable and important works that each of them is written in a specific style. Knowing the characteristics of these sources is very effective in more knowing of the transmitters, analyzing and examining the chain of transmitters of a tradition and resolving their difficulties and problems. As an important and enduring work, the book Mu'jam Rejal el-Hadith, among rejali books, is of great importance in the arena of the knowledge of rejal and knowing the transmitters. Its Usuli, Jurisprudent and scholar author, that during many years was teaching and writing in the field of Fikh and Usul, and by using the traditions of the Infallibles (peace with them), was deducting the divine commandments, in his final years of life, by a great amount of scientific saved matters, written a book in the subject of the knowledge of rejal and recorded an enduring work iv his scientific past.In comparison to other Shi'i rejali books, this work has superior qualifications. Thus, knowing it is a necessity for religious students, especially for those interested in rejal.