There was synthesized a number of nanopowders of metals oxides (Fe2O3, Co3O4, ZnO, and CeO2) in the aqueous extract from Petroselinum crispum leaves. There was performed the physicochemical qualification of the synthesized nanomaterials by the methods of UV spectrophotometry and the scanning electron microscopy. SEM-visualization showed CeO2 powders had a spherical morphology (from 14 nm to 500 nm), Fe2O3 had a plate structure (more than 80 nm), and Co3O4 was presented in small cubic particles with sharp edges (from 20 to 100 nm) stuck together in large units (~ 1 μm), ZnO was characterized by a spherical, oval and hexagonal structure (from 60 to 160 nm). The analysis of cytotoxicity of the biosynthesized powders was carried out as counting dead cells in the roots of T. vulgare stained with the vital dye Evans blue after 14 days of exposure with a preparation in a wide range of concentrations (from 10−1 to 10−5 M). The results showed a dose-dependent increase in cell death at a greater extent on the apex of the root compared to the basal area. A remarkable (over 50%) decrease in the cell viability was recorded after the treatment of plants with Fe2O3 (more than 80%), 10−1 M CeO2 (84.7%), 10−1 and 10−2 M Co3O4 (165 and 70%, respectively). However, a slight increase of viable cells was observed after the treatment with ZnO and CeO2 in dilutions of 10−4 to 10−5 M (by 8% compared to the intact samples). In general, the cell viability of the seedling roots of T. vulgare after the exposure to the biosynthesized nanomaterials increased as follows: Fe2O3<Co3O4<CeO2<ZnO.