Biodiversity is one of the important biological factors in determining water quality and preserving the ecological balance. One of the main drainage systems in Minia is the Orfaily Drain .In this study, variations in both the physico-chemical characteristics and the phytoplankton of Orfaily Drain were monitored over a period of one year (from May 2022 to April 2023). All the water quality variables measured showed considerable monthly variations. Marked seasonal quantitative and qualitative differences occurred in the phytoplankton communities of the drain. The maximum crop density (3676 x 10⁵ taxa l^-1) was obtained in June 2022, while the lowest count (512 x 10⁵ taxa l^-1) occurred in December 2022. Changes in total algal counts throughout the investigation coincided closely with abundance of Chlorophyceae and Cyanophyceae. The major algal groups at this site were Chlorophyceae, Cyanophyceae, Bacillariophyceae, and Euglenophyceae. Fourty two genera were found, including 17 genera of Chlorophycae, 12 of Bacillariophycae, 11 of Cyanophycae, and 2 of Euglenophycae. Cyclotella meneghiniana, Navicula sp., Scenedesmus bijuga and Chroococcus turgidus were recorded in a high rank of occurrence, however other recorded species were be found to moderately, frequently or rarely recovered. In general, the species data indicate that the Orfaily Drain water can be categorized as eutrophic. It was concluded that the phytoplankton groups are very sensitive and helpful indicators to measure the quality of freshwater ecosystems and the health state of any water stream.