In this paper, Biostratigraphy of Asmari formation to San oligo-Myocene was studied in North Mamulan, South West Lorestan Province. In this region, Shahbazan-Asmari carbonate formation is located between Kashkan formation beneath and evaporative formation of Gachsaran as old as lower Miocene in the form of disconformity above. From 180 m real thickness observed in this study, 23 genera and 15 species of benthic foraminifera were identified and studied. These foraminifera were found in four assemblage zones suggesting that this formation dates back to Oligocene (Chattian-Rupelian) and early Miocene (Aquitanian-Burdigalian). Moreover, based on microscopic studies and visual observations, nine microfacies were identified that were deposited in three zones: lagoon, dam, and shallow open marine.