en Toxicology پژوهشي Research Article In recent years, nanotechnology has gained serious attention for diagnosis, prevention and treatment roles. In this study we synthesized nanoceria or CeO₂NPs (cerium oxide nanoparticles) and compared toxicity of cerium oxide powder in nano and bulk forms in two cancerous and one normal cell lines. The cell lines were cultured in a standard humidified incubator, at 37 &deg;C in a 5% CO₂ atmosphere, in RPMI 1640 medium. The cells were incubated with different concentrations of cerium oxide (from 2 &mu;g/mL to 64 &mu;g/mL) in bulk and nano forms.&nbsp; To determine the effect of cerium oxide on cell viability after 24 h, 48 h, and 72 h incubation, a MTT assay was performed using <em>SKBR3 </em><em>(</em>human breast cancer cell line<em>), </em>A431 (Human epidermoid carcinoma cell line) and C2Cl2 (ATCC mouse skeletal muscle cell line) cells. Analysis of variance followed by Sidak post-hoc test, shows the toxicity of nanoceria is significantly deferent from bulk form on three cell lines in this study and is more on cancerous cells in compared to normal cells especially in higher level of concentrations after 24, 48 and 72 hours (All P<0.05). Additionally, the effect of cell lines, cerium oxide forms and concentrations cerium oxide leads in significantly the lowest amount of viability after 72 hours compared with 24 hours and 48 hours. Cerium oxide, Nanoceria, MTT assay, C2Cl2 cells, A431 cells, SKBR3 cells 1 6 http://ijpt.iums.ac.ir/browse.php?a_code=A-10-38-1&slc_lang=en&sid=1 FATEMEH SOLTANI 10031947532846001545 10031947532846001545 No Department of Medical Radiation Engineering, Science & Research Branch, Islamic Azad University, Tehran, Iran KAMAL YAVARI 10031947532846001546 10031947532846001546 No Radiopharmaceutical Research and Development Lab (RRDL), Nuclear Science and Technology Research Institute (NSTRI), Tehran, Iran MAHDI SADEGHI sadeghi.m@iums.ac.ir 10031947532846001544 10031947532846001544 Yes Medical Physics Department, School of Medicine, Iran University of Medical Sciences, Tehran, Iran ALI BAHRAMI SAMANI 10031947532846001547 10031947532846001547 No Radiopharmaceutical Research and Development Lab (RRDL), Nuclear Science and Technology Research Institute (NSTRI), Tehran, Iran SIMINDOKHT SHIRVANI ARANI 10031947532846001548 10031947532846001548 No Radiopharmaceutical Research and Development Lab (RRDL), Nuclear Science and Technology Research Institute (NSTRI), Tehran, Iran