Abstarct: In this study, cerium oxide nanoparticles were prepared by biosynthesis method in the new plant extracts media from cerium nitrate hexahydrate as precursor. In these syntheses, organic molecules of plant extracts as capping agent prevent the growth and agglomeration of particles. The obtained products were characterized by various physical and chemical methods such as TGA/DTA, FESEM equipped with an energy dispersive X-ray spectrophotometer (EDX), XRD, FTIR and UV-Vis spectroscopy. Results showed the morphology of particles was spherical or uniform agglomerated which their size was below 30 nm. The change in calcination temperature and ratio of plant extract to cerium precursor had significant effect on the size and morphology of particles. Cerium oxide nanoparticles could degrade Rhodamine B dye solution under irradiation of 11 W UV light. This photo-catalyst reaction accelerated in neutral conditions in comparison to acidic and basic environment. Investigation of the degradation process mechanism showed that holes are more effective than hyrdroxyl radical and oxygen radical anions. Cellular toxicity (MTT) test for cerium oxide nanoparticles in all concentration showed no significant toxicity on Human lung adenocarcinoma epithelial (A549) cell line. Antioxidant property of cerium oxide nanoparticles was investigated by 2',7'-dichlorofluorescein diacetate (DCFDAH2) test. Results demonstrated that these nanoparticles effectively decreased reactive oxygen species (ROS) produced by cellular mextabolism. Also, type of plant extract used in the synthesis process of nanoparticles due to effect on particles size and ratio of cerium +3 to +4 can affect antioxidant property. Antibacterial property of cerium oxide nanoparticles on negative gram (Escherichia coli) and positive gram (Staphylococcus aureus) bacteria was investigated by minimum inhibitory concentration (MIC) method. Results showed that these nanoparticles had no noticeable antibacterial effect; moreover, in high concentrations they improved the growth of bacteria while in presence of UV light, cerium oxide nanoparticles not only prevented the growth of bacteria but also decreased it at about 30%. Due to evaluate bio-distribution of cerium oxide nanoparticles in rat’s body, nanoparticles were labeled by radioactive Technetium (99mTc). The results of thin laxyer chromatography test showed that labeled nanoparticles were stable with chemical purity more than 95% in phosphate buffer (PBS) and blood serum; therefore, they were proper for in vivo bio-distribution studies. Bio-distribution of labeled nanoparticles in rat during 24 h shown the most accumulation of labeled nanoparticles was in kidneys and a little less in the liver.