A biostratigraphic study based on calcareous nannofossils was performed on the chalky limestone from the Abderaz Formation in the Mozduran section (Kopet Dagh basin, NE Iran). Semiquantitative estimates of total nannofossil and single species abundances showed that calcareous nannofossil assemblages are common and their preservation is good. In this study 37 species have been identified in chalky limestone beds. The following biohorizons were identified: the first occurrence (FO) of Broinsonia parca parca; the last occurrence (LO) of Marthasterites furcatus; the FO of Ceratolithoides aculeus; and the FO of Uniplanarius sissinghii. Based on nannofossil assemblages' four biozones were determined in this section which is the time equivalent of UC14 to UC15cTP of (Burnett 1998), and CC18 to CC21 of (Roth et al. 1986). According to present biozones, early Campanian to early late Campanian ages are suggested. Calcareous nannofossils allow the detailed reconstruction of paleoecological trends in the chalky limestone beds. The chalky limestone of the Abderaz Formation was deposited in a shallow-marine environment, at relatively low latitude with low primary productivity.