This study focuses on paleoenvironmental changes based on abundance of calcareous nannofossils from the Selandian to the Ypresian and Paleocene-Eocene thermal maximum (PETM) interval in the north of Izmit Province. Six biozones were identified from two stratigraphic sections in the AtbaAYA +/- and Caycuma formations in the study area. Paleoenvironmental conditions changed rapidly because relative abundances of Coccolithus pelagicus and Prinsius spp., which were adapted to mesotrophic conditions, and relative abundances of Fasciculithus spp. and Sphenolithus spp., which were adapted to oligotrophic and warm water, were variable in the Selandian. On the contrary, it is seen that oligotrophic and warm water generally dominated in the Belen region. Environmental conditions vary from oligotrophic to mesotrophic in zones NP7 and NP8 in the Thanetian. Species richness of calcareous nannofossil and abundances of Coccolithus pelagicus and Prinsius spp. decreased in subzone NP9a. On the contrary, abundances of genus Fasciculithus spp. Sphenolithus spp. and Discoaster spp. increased in the Thanetian. Abundances of Sphenolithus spp. and Discoaster spp., which were adapted to oligotrophic conditions, increased, while there was not much change in the abundance of Fasciculithus spp. within the PETM interval. In the PETM interval, the abundance of Coccolithus pelagicus, which was adapted to mesotrophic conditions, decreased rapidly, whereas Ericsonia subpertusa disappeared. When the environmental conditions recovered again just after the PETM interval, the abundance of the oligotrophic genus Fasciculithus declined and immediately disappeared, while the mesotrophic species Coccolithus pelagicus, Ericsonia subpertusa, Prinsius spp. and Toweius eminens became dominant in the Ypresian. An increase in the abundance of Discoaster spp. and Sphenolithus spp. continued in the Ypresian.