It is generally thought that the seasonal succession of phytoplankton is minimized during winter months. However, some studies have indicated that there is also diversity in phytoplankton communities in winter. The main purpose of this study was to determine the phytoplankton community structure and species composition of a lake during winter, when covered with ice. Phytoplankton samples from the lake were taken in the winter seasons of 2015 and 2016, during the period from the appearance of the ice cover on the lake until it completely melted, and phytoplankton composition in the lake and some physicochemical properties of the lake water were measured. The phytoplankton community was found to be dominated by Cyclotella meneghiniana, which is a centric diatom, followed by the flagellates, especially Synura uvella, small cryptophytes (Cryptomonas paramecium), dinoflagellates (Peridinium aciculiferum and Gymnodinium sp.), and nonfilamentous greens (Pediastrum duplex, Scenedesmus spp., and Monoraphidium spp.). Phytoplankton development under ice-cover is largely related to temperature, but the development of phytoplankton composition is random. A low correlation was determined between the dominant organisms and ice thickness. Species biodiversity was low, but the dominant species started to be represented with different taxonomic groups after mid-winter.