Phytoplankton life pattern based on size, shape and available nutrient was classified in three major strategies namely R (high S/V ratio, resistance to water mixing), C (medium to high S/V ratio, invasive growth and high competitors' species) and S (low S/V ratio, resistant to stratified water and nutrient deficiency). Seasonal sample collected by Niskin in eight transects perpendicular to the coast at depths of 5, 10, 20, 50 and 100 meters at different layers (surface, 10 and 20m) in 2009-2010 from Iranian coast of the Caspian Sea. The objective of the survey is determination of dominant phytoplankton species strategies with emphasis on size (surface to volume ratio) as morphology parameter. Important Species Index (ISI) of Exuviaella cordata, Chrysochromulina sp. (C-Strategy) and Oscillatoria sp. (R-Strategy) were high in spring. This value was for Oscillatoria sp. and Thalassionema nitzschioides (R-Strategy) in fall. In winter, high ISI belonged to Pseudonitzschia seriata, Cerataulina pelagica, and Dactyliosolen fragilissima (R-Strategy). Finally in summer, Oscillatoria sp. (R-Strategy) was the predominant species. Multivariate analysis (CCA) showed the interaction between environmental parameters and species presence as well as role of morphology in the phytoplankton communities’ without their phylogenic relationships. This research, as first attempt in the Caspian Sea, showed the applied phytoplankton morphological studies in determining the ecological status and management of water resources.