Selenium nanoparticles (SeNPs) are at the center of some intense biomedical researches due to their high biological activity and significantly lower toxicity compared to other forms of elemental selenium or its compounds. These particles are appreciated for their ability to annihilate free radicals in vitro and in vivo. In vivo effects are not only reduced to antiradical activity, but are manifested by numerous benefits for macroorganism, such as improving growth and reproduction performance. Currently, there are results demonstrating that biosynthesis of selenium nanoparticles can be achieved using different cultures of microorganisms or extracts from plants and algae. It is also shown that the coating of selenium nanoparticles with different organic molecules (amino acids, polysaccharides) leads to enhance their biological effects. In this study, a synthesis system was used to produce selenium nanoparticles containing sodium selenite and polysaccharide fraction derived from biomass of cyanobacterium Spirulina platensis. In bionanosynthesis system involving selenium nanoparticles and polysaccharide extract from spirulina was established that changing the amplitude of absorption peaks started to rise after 30 min, which means the accumulation of a significant number of nanoparticles in the system. Over a time course of 120 min from the initiation of the reaction, a steady increase in the maximum absorption wavelength was observed, indicating the possibility of enhancing the yield of biosynthetic system at the expense of increased contact time. It was determined the modification of reducing power of fractions depending on their contact time with sodium selenite. Reducing power of polysaccharide extract was 0.11 mg ascorbic acid per ml extract (equivalent), and after contact with selenium ions for 120 minutes it decreased by 89.1%. This parameter has decreased by about 22% compared with the period of 90 min contact. Thus, biosynthesis of selenium nanoparticles on polysaccharide fractions derived from spirulina biomass was allowed to continue for 120 min followed by decreasing the reducing power of solution, which indicated the possibility of continuing the process.