The dried cells of microorganisms are known as single cells proteins, which used as protein supplement in food or animal feeds. Microorganisms such as Yeast, Bacteria, Fungi, especially algae utilizing CO₂ and sun light as source of carbon and energy for growth to produce biomass. Spirulina is blue green mesophilic cyanobacteria with high protein content being largely used as single cell protein. Single cell protein used in diets for weight reduction and malnutrition. It has highest protein content, high vitamin value particularly B12 and lipid content, used as antioxidant and in medicine. The present review reports the microorganisms as source of single cell protein with special reference to Spirulina maxima.