This study was carried out to evaluate species composition and distributions of phytoplanktonin relation to changes in some physicochemical characteristics of water in Dadinkowa Dam, Gombe State, Nigeria. Samples were collected using plankton net by hauling for five meters. Samples were kept in plastic containers and preserved in 4% formalin and logols iodine solution. 1 ml of sub-sample was observed under microscope. Physico-chemical parameters recorded include water temperature which ranged between 28 O C and 31.9°C, pH ranged from 7.1 to 8.9, Transperancy ranged from 0.06m to 0.16m, Dissolved oxygen range from 1.7 to 3.9mg/l and Conductivity ranged from 81 to 100µs/cm. 22 species of phytoplanktons were recorded and the maximum mean density was 92,105 cell/L and minimum was 44,317 cell/L. The dominant phytoplankton was Bacillariophyceae38.1%, followed by order Chlorophyceae 35.4%, Myxophyceae25.3% and Chrysophyceae 1.22%.(The phytoplankton varied significantly with months of sampling.