Calcareous nannofossil assemblages recovered from a long, continuous section have been described from the Sarayan region in Lut Block in east Iran. The marine Upper Cretaceous sediments of this section yield medium diverse and well-preserved calcareous nannofossil assemblages. A structural analysis of Upper Cretaceous calcareous nannofloras has revealed 30 nannofossil species. A revised zonation for the Cretaceous is used to subdivide the Cretaceous deposits in this section into five zones (CC21-CC25). Finally, the paleoecological applications of Upper Cretaceous nannofossils are considered in this section. Surface water temperature, productivity, and fertility are believed to have been the principal factors controlling species distribution. Warm water indicators such as Uniplanarius sissinghii, Uniplanarius trifidus, and Micula murus suggest warm surface water conditions in the studied thickness. In the Sarayan section, based on Lithraphidites carniolensis and Watznaueria barnesae, lower fertility conditions with low productivity at the Campanian to Maastrichtian were suggested for this region.