ABSTRACT: The calcareous nannofossil genus Reticulofenestra and its related taxa are called “reticulofenestrids,” and they were one of the major nannofossil groups during the Cenozoic. The reticulofenestrid specimens are composed of two shields, proximal and distal, which are circular or subcircular in shape and consist of tiny calcite plates. Cyclic changes in the size and shape of reticulofenestrid coccoliths have been used to characterize their evolution. The major taxonomic turnover in this group was related to environmental changes during the Cenozoic. From the Oligocene to the Miocene, there were several different types of reticulofenestrids. The primary taxon of reticulofenestrids disappeared during the Oligocene-Miocene boundary, and elliptical species emerged during the early Miocene. However, it is uncertain exactly when and how the change in reticulofenestrids occurred. To understand the evolutionary trends of reticulofenestrids, we conducted biometric analyses from the Oligocene to the Miocene. Detailed observation and measurement of reticulofenestrid coccoliths under a scanning electron microscope revealed a significant size reduction in the group near the Oligocene–Miocene, which corresponds to a period of rapid global cooling. Furthermore, regarding the reticulofenestrid taxa in this study, central structures characterize the Oligocene genus Dictyococcites, and the long axis length of the coccolith outline and central areas can be effective characteristics for distinguishing between theMiocene Cyclicargolithus and Reticulofenestra. These results suggest that morphological differences of reticulofenestrids are expressed not only in the shape and size of the coccolith but also in the size and shape of its central area.