Since the origin of life, the most primitive organisms attempted to adapt in different climatic condition and became successful in their survival in adverse as well as congenial environment. In this backdrop, the present analysis on selected fossil marine phytoplankton and zooplankton has been taken into consideration from the sediments of northeast Indian Ocean ranging in age from ~16.4 million years old (Miocene Epoch) to ~4.19 million years old (Pliocene Epoch). Morphometric analysis of selected seven species of significant phytoplankton—calcareous nannofossils has been performed. Twenty randomly-selected specimens of each species were studied in each sample. The data was imported into the software PAST, where box-plot diagrams were created in order to observe possible variations in the size of the specimens. Several climatic events during Miocene to Pliocene have been earmarked. It has been visualized that at least seven different species of calcareous nannofossils continued their existence during this period. However, variations in size have been observed in those species and their gradual change in size have been studied. The shells of marine zooplankton radiolarians are composed of opaline silica and their shell volume plays an important role for their adaptation and diversity. The shell volumes of five suitable nassellarian and one spumellarian radiolarian species recovered from the sediments of deep-sea drill cores have been measured from two dimensional microscopic studies using a mathematical formula. The volume of silica used by individual radiolarian species was estimated by considering as hollow spheres or cones. Both these studies from the northeast Indian Ocean is a new approach as it was not attempted earlier from this region. These changes observed both in phytoplankton size and zooplankton volume can be attributed to different climatic events as well as their evolution in time and space.