In this work, several nanostructures (nanopowders and nanostars) of undoped and 1%, 3% and 5% europium (Eu3+)-doped ZnO have been synthesized via coprecipitation method using oxalic acid and sodium hydroxide as precipitation agents. Starting from zinc acetate and europium acetate, nanopowders were obtained by coprecipitation with oxalic acid. ZnO based nanostars were synthesized by coprecipitation of Zn2+ and Eu3+ with hydroxide ions (HO-), when zinc chloride and europium acetate were used as reagents. The structure and morphology of the as-prepared ZnO nanopowders and nanostars were investigated by X-ray diffraction and electron microscopy. Only würtzite structure of ZnO was identified in all the samples based on ZnO. Transmission electron microscopy (TEM) investigations have shown an average particle÷crystallite size range from 23 to 29 nm and polyhedral and spherical morphology with tendency to form aggregates for nanopowders. Cytotoxicity tests on MG-63 cell lines was also performed. Photocatalytic activity of ZnO nanopowders have reached higher values compared to ZnO nanostars. The photocatalytic test indicates that the ZnO nanopowders have better activity than the nanostars, most probably because of the higher specific surface. Doping the ZnO with Eu2O3 does not seem to alter it in a decisive manner. The toxicity results indicated that ZnO nanoparticles (NPs) high toxicity on tumoral cells is also induced by particle size and, consequently, the dissolution of Zn2+ ions is dependent on the size of the particles, increasing with the particles size.