Today, in the nanotechnology world, considerable interest has been focused on the green synthesis of non-toxic and environmentally friendly nanoparticles without the use of chemicals that are often expensive, toxic and dangerous. In the biosynthesis of nanoparticles (NPs), chemicals reducing agents are replaced by extract of various organisms that contain bio reductive compound as stabilizing and capping agent for NPs. Furthermore, it is possible to control the size and shape of nanoparticles using different concentrations of biological compound as reducing agents. In this study, bismuth oxide nanoparticles (Bi2O3 NP) were prepared via green synthesis using cyanobacterial cell extract of Spirulina platensis. The Physico-chemical properties of the Bi2O3 NP characterized by X-ray diffraction (XRD), energy dispersive X‒ray (EDX) analysis, Fourier-transform infrared (FT-IR) spectroscopy, scanning and transmission electron microscopy (SEM). In conclusion, our results revealed that Spirulina platensis can be used as a good organism for biologically synthesis of Bi2O3 NP.