Abstarct: Fajan Formation with the Paleocene age has been studied in the Mojen outcrop in the northwest of Shahroud in Semnan province. Fajan Formation is a collection of siliciclastic rocks including shale, sandstone and conglomerate, which is 977 meters thick in this sequence and is divided into upper and lower members. The Fajan Formation in this section consists of 16 lithostratigraphic units, which mainly include shale and sandstone in the lower member and shale and conglomerate in the upper member. According to facies studies, 7 rock facies including coarse-grained (Gcm, Gmm, Gh), medium-grained (Sh, Sm, Sp, St, Sl, Se) and fine-grained (Fm) facies were identified. baxsed on this, the structural elements of GB, CH, SB, FF, CS have been identified that the lower member is related to the meandering river environment and the upper member is related to the braided river environment. The petrography of thin sections and the results of modal analysis show that the sandstone samples are litharenite of the calclithite type (23 samples), feldspathic litharenite (5 samples) and lithic arkose (1 sample). Diagenesis studies in different sandstone petrofacies are represented the various processes including feldspar alteration, longitudinal and point contacts, calcite and iron oxide cements, equant and poikilotopic calcite cement, siliceous and feldspar overgrowth cement, fracture and filling the grains by calcite cement. Due to the presence of abundant carbonate rock fragments with large size and significant amount of angular quartz grains without overgrowth cement, it is possible to suggest the first cycle and short transportation for these sediments. Also, the presence of quartz grains that have abraded overgrowth cement can also indicate the recycled origin of some detritus in this formation. baxsed on the modal analysis, the climate of the source area is semi-arid to semi-humid, and geochemistry and determination of weathering indices also indicate moderate weathering in the source area. Geochemical studies also showed that Fajan Formation sandstones are placed within two tectonic environments of rift and collision.