A PRELIMINARY Checklist on phytoplankton dynamics of the Rabigh Dam Stream was investigated during the wet (January and February) and arid (July and August) seasons of 2018. Composite sample were collected along the Rabigh Dam Stream. These outcomes provide useful information on the checklists and ecology of plankton's species which could be potentially used as bio-indicators for assessing and monitoring the Rabigh Dam Stream. Five main algal groups were recorded namely: Cyanophyceae, Chlorophyceae, Euglenophyceae, Bacillariophyceae and Pyrrophyceae. This study identified 65 phytoplankton species consisted of Blue-green algae (9 species), Green algae (46 species), Euglenoids (1 species), Diatoms (6 species), and Dinoflagellates (3 species). The following order of dominance was; Cyanophyceae> Chlorophyceae> Euglenophyceae> Bacillariophyceae> Dinophyceae. In spite the number of blue-green algal species was low, they represented 63.62% of the total phytoplankton followed by Chlorophyceae (19.08%), Euglenophyceae (11.21%), Bacillariophyceae (5.72%), respectively. On the other hand, the Dinophyceae represented the rarely group (0.37%) of the total phytoplankton. There were main changes in phytoplankton composition due to the projects drinking water reservoir construction. It could be related to the severe human activities and difference in sampling strategies. It was concluded that all the dams had well balanced phytoplankton communities. The changes in phytoplankton composition and numbers were significantly varying among the period of study.