Europium oxides (Eu2O3) nanoparticles are extensively employed in diverse areas ranging from bioimaging to chemical sensing. However, still there is significant lack of information regarding their toxic behaviour on humans and the environment. The endeavor of this work is to develop structural/toxicity correlations for bare and surface functionalized Eu2O3 nanoparticles synthesized by low-temperature colloidal route. The detailed investigations revealed that Eu2O3 nanoparticles concentration have significant effect on the in vitro toxicity. On the basis of antibacterial, antimicrobial assays and assessment of the genotoxicity using Allium cepa and molecular studies on human blood DNA, it was discovered that surface modification of nanoparticles is an important parameter for reducing the toxicity of Eu2O3 nanoparticles. The phytotoxicity of the prepared nanoparticles has also been compared by the seed germination assay with wheat, green grams, and black chickpeas. The detailed results revealed the mechanism of Eu2O3 nanoparticles in vitro toxicity. © 2018