Cyanobacterial blooms deplete nutrients, reduce water clarity, exhaust carbon di oxide and produces secondary metabolites which negatively affect aquatic organisms and water quality. Control of algal blooms using metal nano particles is one effective method for the safety of water environment. Cobalt nano particles (CoNPs) were synthesized and tested against microalgae isolated from fresh water cyanobacterial blooms by assessing the effects on growth rate, biomass concentration, photosynthetic pigments concentration and antioxidant enzyme activity. Microcystis and Oscillatoria were identified as the predominant isolates from algal blooms and treated with varying concentrations (1, 2, 3, 4 and 5 mg·L -1 ) of CoNPs. Steady decline in the growth rate of microalgae was observed at the end of 5 days indicating the toxicity of CoNPs on microalgal growth. At the end of cultivation period, 78% and 88% of reduction in biomass concentration of Microcystis and Oscillatoria were observed at 5 mg·L -1 of CoNPs. The chlorophyll content was reduced from 1.53 to 0.24 mg·L -1 in Microcystis and 1.63 to 0.29 mg·L -1 in Oscillatoria. There was a 69.3% and 73.2% decrease in carotenoid content of Microcystis and Oscillatoria respectively. Both protein and carbohydrate contents of the microalgae were reduced with increasing concentration of nano particles. The decrease in Super oxide dismutase (SOD) activity with increased nanoparticle concentration reveals the formation of stress in the microalgae. The increasing GSH activity proved the effect of CoNPs on the activation of antioxidative enzymes to protect the cells. This study demonstrates the efficiency of cobalt nano particles (CoNPs) on inhibition of fresh water algal blooms thereby reducing the eutrophication problem.