Calcareous nannofossil assemblages of the boundary Paleocene-Eocene deposits (Pabdeh formation) of the Zagros, Dezful Embayment-Mansouri oilfield have been studied in detail. It consists of shale and grey marlstone with inter layers of calcareous argillites and marly claystone. We present a study focused on changes in calcareous nannofossils of the Pabdeh formation during the Paleocene/Eocene Boundary (PEB). As a result of this study, 15 genera and 24 species of nannofossils have been identified for the first time. The abrupt increase in genus Discoaster and Tribrachiatus abundance provide distinct data for quick recognition of the Paleocene/Eocene Thermal Maximum (PETM) warming event. The evolutionary changes among the nannofossils created two distinct biostratigraphic marker zones in PEB, described as nannofloral zones NP9 and NP10. According to these zones, the age of boundary is Late Paleocene (Thanetian)-Early Eocene (Ypresian).