Abstarct: In some parts of the Alborz Zone at the baxse of the Shemshak Formation, there are a series of mafic igneous rocks in the form of lava or intrusive masses that have been reported by numerous geologists in different regions of the Alborz, including the Eastern Alborz. The intrusive mass studied in this region are two differentiated Sills, each with a thickness of about 50 m, which are located among the shales and sandstones in the lower part of the Shemshak Formation. The upper and lower parts of these Sills have child margins and are very fine grained. Gheshlagh Sill igneous rocks include differentiated rocks such as Olivine-Gabbro, Gabbro-Gabbrodiorite, Diorite, Monzodiorite-Monzonite and Syenite. These rocks show granular, intergranular, and ophitic textures and are composed of the main minerals of olivine, feldspar (plagioclase and alkali feldspar), clinopyroxene and amphibole. In the petrological and tectonomagmatic discrimination diagrams, the alkaline nature of the magma and intracontinental plate setting of these rocks, have been proved. In the the Chondrite and primary mantle normalized and multi-element diagrams, enrichments in the LREEs and LILEs and depletion in the HREEs and the absence of Eu anomaly are observed. The chemical composition of the clinopyroxenes of the Gheshlagh igneous rocks is often of the augite type and plott in the field of the iron-magnesium-calcium and poor of sodium pyroxene group (Quad). Applying the thermobarometery calculations on the clinopyroxenes have been resulted to the temperatures of 1100 to 1155 °C at the pressures of 5 to 10 k bar for crystallization of this mineral. The composition of the feldspars of these igneous rocks are in the range of Ab20.91 An79.09 – Ab99.52 An0.48 (labradorite- bytwonite) and Ab1.39 Or98.61 - Or0.12 Ab99.88 (albite- anorthoclase). The amphiboles of these rocks are in the group of the calcic amphiboles with actinolite, hornblende, magnesiohornblende, tchermakitic hornblende to tchermakite, and tectonically plott in the field of the intra-plate amphiboles.