Nowadays, the synthesis of metal nanoparticles using marine algae is a growing technology. This method has become more common than previous physical and chemical synthesis technologies. Because natural products are used during the synthesis, and as a result, it will be less toxic for humans and the environment. Also, in this case, features such as shape, size, composition and stability of nanoparticles, which all affect their bioactive properties, are superior when physical and chemical methods are used. Also, the abundance of algae as a raw material reduces the costs of green synthesis. In this study, bismuth oxide nanoparticles (Bi2O3) were produced by green synthesis method and using Gracilaria algae cell extract. Characteristics of synthesized nanoparticles using tests; X-ray diffraction (XRD, Fourier transform infrared spectroscopy (FT-IR), visible-ultraviolet spectroscopy (UV-VIS) and transmission electron microscopy (TEM) were investigated. Our results showed that Gracilaria algae is a suitable species for the green synthesis of Bi2O3 NP.