This paper examines in detail the little-known Friday mosque (masjid-i jāmi‘) of Firdaus, located southwest of the Khurasan, which has had a key role in developing Saljuq architecture in the area. The original scheme of the mosque was unclear. The lofty īwān and two flanking dome chambers on the west (qibla) side of the courtyard are the central core of the mosque. The main aim of the article is to analyse the architecture of the mosque and demonstrate its association with the architectural style of the Saljuq mosque in the Khurasan area. During the conservation and restoration measures in 2006, some fresh materials were unearthed. The new findings shed light on the original features of the mosque and revealed its formation. The paper describes the city's history, defines its architectural characteristics, and then analyses the present information for replying to the research questions. Owing to the outcomes of the archaeological investigation, the study suggests the general scheme of the mosque as a further example of the two- īwān mosque, which may be dated to the late years of the 6th /12th century. Despite the importance of the mosque, no lengthy study has been published about the building. The accurate drawing of the plan and sections of this monument, with the proposal scheme for the reconstruction of the mosque, are being published in this paper for the first time