Two field surveys on Phytoplankton were conducted in 2012 at 26 sitesof Dau Tieng irrigation system. Resulted from the study, a total of 329 species of phytoplanktonbelonging to 125 genera, 69 families, 35 orders, 11 classes of seven phyla were recorded. The species composition included both types of species origins: The freshwater origin (e.g., species of Chlorophyta and Euglenophyta) and the marine origin which invaded into inland waters (e.g., species of Bacillariophyta and Dinophyta). Among four surveyed areas, the Day Man preventive saltwater area has the highest species diversity and the drainage channels has the lowest species number. Species richness of phytoplankton is higher in dry season than in rainy season, however, the structure of species composition and the number of predominant species are not significantly changed.