The Antarctic ecosystem is one of the most sensitive world regions to the climate change, and exhibits a wide environmental variability that modifies the temporal and spatial patterns of the phytoplankton structure. The phytoplankton community and its spatial and temporal variability of Mackellar inlet-Admiralty bay, Antarctic, were studied in two austral summers (years 2012 and 2013). Samplings were done in 11 fixed stations at three depths (0, 10 and 20 m). Antarctic Surface Water mass was identified in the surface layer at the study area. Phytoplankton composition in the water column consisted in 40 diatoms taxa, 6 dinoflagellate taxa and various species of nanoflagellates (< 20 µm). Nanoflagellates from the Mackellar inlet included Prasinophytes, Cryptophytes and Haptophytes species. Diatoms of the microplankton fraction (20-200 µm) and nanoflagellates (< 20 µm) were in alternation during the period of this study: by summer 2012 nanoflagellates were more abundant (86 %), with diatom species of the genus Thalassiosira (11%), and by summer 2013 phytoplankton abundance was dominated by microplanktonic diatoms such as Thalassiosira delicatula (36%), Pseudo-nitzschia group delicatissima (9%), Porosira glacialis (6%), the athecate dinoflagellate Gymnodinium sp. (19%) and nanoflagellates (20%). Cell densities were lower in summer 2012 compared with those in summer 2013, where a diatom bloom occurred. Phytoplankton diversity was relatively low (< 1.97 bits⋅cell-1) in both summers. The main differences between the two sampling periods were the dominance of microplanktonic diatoms and nanoflagellates, which were probably caused by the decrease in salinity and increase in subsurface dissolved oxygen in the study area produced by ice melting in summer 2012 and 2013.