The Kopeh Dagh sedimentary basin is located in northeastern Iran, where sedimentation was continuous throughout Jurassic to Tertiary times. This basin is one of the promising areas for hydrocarbon exploration and contains giant gas fields such as Khangiran and Gonbadli (in the east Kopeh Dagh). The Sarcheshmeh formation (Aptian) is one of the rock units from the Kopeh Dagh basin and composed mainly of marl, shale, and limestone. The purpose of this study is to evaluate the potential of hydrocarbon generation of the Sarcheshmeh formation as a hydrocarbon source rock. For this purpose, 40 samples were collected from the outcrop of this formation in the eastern part of the Kopeh Dagh (Khomari section) and 20 of them evaluated using Rock-Eval pyrolysis and organic petrography (Vitrinite reflectance measurement and visual kerogen analysis) in order to characterize the content, type and the thermal maturity of the organic matter. Also, to assess the source rock potential more accurately, Rock-Eval pyrolysis data were used from six cuttings and 86 outcrop samples of the Sarcheshmeh formation in the western areas of Kopeh Dagh. The results showed that the total organic carbon values of most of the studied outcrop samples are very low (TOC 0/5%) and are not to be accepted as a source rock. However, the samples of Sogha and Takal Kuh sections (the westernmost outcrops of Sarcheshmeh formation) display higher TOC values (TOC 0/5%), but the organic matter has a low hydrocarbon generation potential as indicated by low S2 values. Rock-Eval pyrolysis shows that the organic matter in the Sarcheshmeh formation is mainly composed of Type IV kerogen. Also, organic petrographical observations for samples of Khomari section indicate that organic matter in these rocks is classified as coaly kerogen and confirm the concluded results of geochemical data. Rock-Eval T max shows that the majority of samples from western parts of the basin are in the immature to the early mature stage of the hydrocarbon generation. In contrast, mean vitrinite reflectance values (R o ~ 1.1-1.3%) and spore coloration index results (SCI ~ 7-8.5) indicate organic matter in all the samples of Khomari section is thermally mature. In addition, results obtained from the analysis of six cutting samples indicate that organic matter in these rocks has a fair to good hydrocarbon generation potential and is classified as type II/III and Type III kerogen. T max shows that most of these samples are in the immature to low mature stages of the oil generation window.