In order to evaluate the source rock potentiality of the Garau Formation, 14 outcrop samples were collected and subjected to preliminary geochemical assessment, from Tange-Shanak valley south of Aligoodarz city. Primary results from Rock - Eval II pyrolysis, indicate that; this formation contains sufficient TOC values. TOC contents range between 0.88 to 5.71%.The quality of organic matter in terms of hydrogen index (HI) between 298 up to 610 (mg HC/g Total Organic Carbon), with mainly kerogen type II, with small contribution of type I kerogen. Maturity level of organic matter in this Formation is based on (Tmax), is between 421 to 443 °C. This formation exhibits a good source rock potential that has entered Oil windows stage, with capabilities of producing liquid (crude oil) hydrocarbons. Based on HI vs OI plot, the origin of organic matter of the Formation is marine, which is deposited in reducing environments. With help of pyrolysis results and comparison of hydrogen and oxygen contents of samples, organic facies and depositional environment were determined. Based on HI and OI indices, the organic matter of the Garau Formation originated from marine algae is composed by organic matters with terrestrial origin.The studies on the Biomarkers of examined samples show that the origin of the organic matter of this Formation usually belongs to open marine, however sometimes the trace of organic matter with deltaic origin is observed by entrance of some terrestrial organic matter. The studied samples have been evaluated by using the GC-MS technique with comparing C29/C30 against C34/C35 Hopanes, the type of the lithilogy of formation is clastic (shale) lithology and the rest little amount of them is carbonate type.