The hydrobiology of the Caspian Sea has been systematically studied since 1934. Results of the present study showed that phytoplankton of Southern Caspian Sea include 193 species from 5 phyla, including 92 species of Chrysophyta, 45 Cyanophyta, 23 Pyrrophyta, 21 Chlorophyta and 12 Euglenophyta. Comparison of phytoplankton before and after M. leidyi invasion of Caspian Sea showed that phytoplankton species have increased over time.Although the abundance of Chrysophyta increased, but in terms of biomass, Chrysophyta showed a sharp decrease. This is because member of Chrysophyta was dominated by smaller species such as Nitzschia seriata (an aggressor species). The Pyrrophyta and Cyanophyta abundance and biomass decreased sharply. On the other hand, abundance of Chlorophyta increased. It is most likely that a sharp decrease in zooplankton predation due to their decrease in numbers as mentioned by previous workers (as a result of M. leidyi predation) that caused an overall increase in phytoplankton of southern Caspian Sea after invasion by the comb jelly (M. leidyi).