Cerium oxide (CeO2) nanopowders were successfully synthesized by a simple microwave-hydrothermal method. The UVB protection of an emulsion containing CeO2 was evaluated in vitro and compared to the one provided by the same emulsion, zinc oxide (ZnO) instead of CeO2. CeO2 offered better protection against UVB radiation. Finally, in vitro cytotoxicity of CeO2 and various ZnO commercial sunscreens was carried out using NCTC2544 cell line. Assessment of cell viability demonstrated that all tested ZnO commercial sunscreens reduce cell viability in a dose- and time-dependent manner whereas CeO2 particles did not produce any effects on NCTC2544 growth at the highest concentration tested (80 μg/ml).