Diatoms from Middle Miocene deposits of Khanka Lake’s northwestern shore (Primorye) were studied using light and scanning microscopes. The analyses of diatom flora composition and species diversity showed that there was a dominace of several taxa that made possible the recognition of two diatom complexes. The lower complex differs from the upper one by higher species diversity, by presence of many subtropical diatoms and absence of pronounced dominant species. Differences between the complexes can be explained by the progressing climatic cooling and predominance of fluvial-lakustrine sedimentation over the typically lake one due to development of a wide system of river valleys.