Maroon Dam Lake is an area with a capacity of ​​1,200 million mm³ of water and a length of 30 km that is located in the Zagros Mountain Valley, 19 kilometers northeast of Behbahan. This study was carried out from spring 2016 to winter 2017 in order to recognize qualitative and quantitative changes of phytoplankton species. Physicochemical properties of water such as temperature, dissolved oxygen, pH, EC and TDS were measured in each sampling in 5 stations. In this study, 42 genera of phytoplankton belonged to 5 phyta (Bacillariophyta, Cyanophyta, Chlorophyta, Euglenophyta and Pyrrophyta) were identified. The average annual density of phytoplankton was 34765±6261 cell/ml^-1. Bacillariophyta with the density of 15465±765 cell/ml^-1 and the abundance of 44.48% and Pyrrophyta with the density of 550±478 cell/ml^-1 and the abundance of 1.58%, respectively had the most and the least annual density of phytoplankton which had a significant difference in comparison with other phyla (P<0.05). Biodiversity indices (Shannon, Simpson and Evennes) showed significant difference between the seasons (P<0.05) but there was no significant difference between stations. The highest Shannon and Simpson diversity incidence was observed in autumn and its lowest level was observed in summer (2.8 and 0.92, 1.1 and 0.46). Species diversity has been reduced in summer due to high temperature, decrease of the surface of water and high evaporation. The results showed that there was a negative correlation between the density of phytoplankton and the investigated factors, but there was no significant correlation between them. The highest negative correlation was observed between pH and TDS with the frequency of phytoplankton. The Maron Dam Lake is not rich in phytoplankton density, probably due to the large amount of water to be emitted per year for farming and the presence of several species of fish in water.