Abstarct: Sefid Sang granitoid pluton (SSGP), with approximately 53 Km2 extensions, is located in 40 Km south of Biarjomand (southeast Shahrood – in Semnan province). This granitoid pluton and its host rocks are a part of the northern margin of Central Iran structural zone and include some parts of old granitic, gneissic, and regional mextamorphic terrains with late Neoproterozoic to early Cambrian age. Field observations and petrographical studies indicate that the compositional rang of SSGP composed of granite, alkalifeldspar granite and leucogranite. This pluton intruded by a numerous diabasic (microgabbroic-microdioritic) dikes with northeast – southwest general trend. Since these dikes in studied area and neighboring areas with similar historical geology such as Delbar and Band e Hezar Chah intruded in mextamorphic rocks of Precambrian and granitoids of late Neoproterozoic - early Cambrian and also detrital sedimentary sequence mainly composed of conglomerate and sandstone of late Triassic – Early Jurassic, and absence of them in the limestone - sandy limestone sequence of upper Jurassic of eastern Alborz zone (Dalichy formation), Therefore they have very younger age in compared with SSGP and the other similar plutons, and also they have completely different origin and nature. With attention to carried out petrographical studies plagioclase, orthoclase and quartz are major minerals and apatite, sphene, allanite and zircon are minor minerals of SSGP. Dominant texture in granitoidic rocks is granular. Diabasic dikes show microgranular, ophitic, subophitic, porphyroidic and microlitic textures. Deformation is typical phenomena in granitoidic rocks and is from high temperature to medium temperature subsolidus types. Negative anomalies of Ti, P, Nb and enrichment of LREE and incompatible elements of Rb,Th and U, and also the presence of mextapelitic enclaves and the abundance of biotite in granites indicate that these rocks have been generated from partial melting of mextagraywack rocks and have weakly peraluminous and subalkaline S-type nature. baxsed on the geochemical diagrams, diabasic dikes have alkaline nature. They are relatively enriched in LREE and LILE, and depleted of HREE and HFSE. With consideration of the positive anomaly of Pb, K, U and negative anomaly of Nb, these dikes have been subjected to crustal contamination. Magma forming of the mentioned diabasic dikes have been generated by 10-25% partial melting of a spinel lherzolitic source in a depths less than 80 Km. Regional geology evidences indicate that studied dikes have been produced in a tensional pull apart basin environment in relation to the subduction zone of Neotethys oceanic lithosphere beneath the Central Iran continental slab. Keywords : Sefid Sang, Shahrood, Biarjomand, S-type granite, Diabasic dike, Crustal contamination.