Analyzing the titles of Ahl al-Bayt (Infallible Imams) is very important. The present article seeks to examine the title Hujjat Allah in terms of the possibility of its exclusiveness to Imam Mahdi. This research, through using Hadith sources, attempts to firstly state that the mentioned title is used for all the Imams; but some scholars, like the late ˈAllāmah Majlisī say that this title is exclusive to Imam Mahdi (pbuh) because he is the last proof of Allah the Almighty on earth however a collective aspect is also mentioned that if there is a presumption to a certain infallible Imam, it is clear that this title will be specific to that Infallible person, but if it comes in a generic term, it will refer to Imam Mahdi (pbuh) however, we do not have any narration based on this matter. though this conclusion is weak, the weakness has been compensated by the practical way of scholars.