Nanotechnology deals with the production and usage of material with nanoscale dimensions (1- 100 nm). The nanoscale dimension provides nanoparticles with a large surface area to volume ratio and thus very specific properties. Metal oxide nanoparticles, such as zinc oxide (ZnO), a promising photocatalysts for the photocatalytic degradation of contaminants in wastewater, and also used in biomedicine, especially in the fields of anticancer and antibacterial fields. In this study, we investigated the synthesis of ZnO-NPs using an endemic plant called Rumex abyssinicus, locally known as Mekemko, for the photodegradation of malachite green dye in the photocatalytic process and antibacterial activity. ZnO nanoparticles were synthesized using a sol-gel method with the addition of an ethyl acetate extract from the plant's root and then subjected to calcination at different temperatures (300 oC, 400 oC, and 500 °C). The prepared ZnO nanoparticles were characterized by UV-Vis, FTIR, XRD, and TGA. The XRD pattern confirmed the hexagonal crystalline phase structure of the synthesized ZnO NPs calcinated at 300 oC, 400 oC, and 500 oC with an average crystal size of 19.87 nm, 26.7 nm, and 28.6 nm respectively. The results of the UV-Vis of ZnO-NPs displayed a strong peak at 290 nm for all ZnO NPs produced at different calcination temperatures. In addition, the FTIR result showed a characteristic absorption peak at 435 cm-1 , 454 cm-1 , and 475 cm-1 for ZnO-NPs calcinated at 300 o ,400 oC, and 500 oC respectively for the presence of Zn-O stretching, and additional peaks also appeared from the plant source, suggesting that the natural functionalities are present together on ZnO particles as a capping agent. The TGA result confirmed the existence of phytochemicals that serve as capping agents and the synthesized ZnO-NPs were calcinated at,400 oC, and 500 oC thermally stable above 613 °C and 634 oC respectively, but ZnO calcinated at 300 oC thermally stable 520 oC. The study also investigated the effects of various parameters, such as calcination effect, catalyst dosage, dye concentration, and pH, on the dye’s photodegradation. The prepared ZnO nanoparticles calcinated at different temperatures were efficiently utilized for the photocatalytic degradation of malachite green dye and it was observed that the samples calcinated at 300 °C, 400 °C, and 500 °C the percentage photodegradation was 88 %, 90 %, and 95 %, respectively. The results showed that 500 oC calcinated ZnO nanomaterials synthesized removed 95 % of the dye from the aqueous solution by photo-degradation under visible light in 70 minute. Moreover, a higher catalyst dosage resulted in a higher dye degradation percentage, with 98.7 % iii degradation achieved using 0.15 g of the photocatalyst in 70 minute. Increasing the pH of the medium also enhanced the dye degradation, with 99% degradation achieved at pH =10 in 70 minute. However, the photodegradation rate decreased with increasing dye concentration. The kinetic investigation of the photocatalytic MG dye degradation has fitted the pseudo-first-order kinetic model. More than 80% removal efficiency was observed after five reuse cycles, proving good stability and reusability of the catalyst. The antibacterial activity of the ZnO-NPs calcinated at 300 oC, 400 oC, 500 oC, and the root extract was assessed by the disc diffusion method against four different bacterial strains. The results showed that the ZnO-NPs calcinated at 300 oC have the highest antibacterial activity against all tested bacterial species with a maximum ZOI of 13.6 ± 0.57 mm compared to ZnO samples calcinated at 400 oC, and 500 oC with a maximum ZOI of 12.6 ± 0.57 mm and 11.6 ± 0.0 mm respectively with the root extract maximum ZOI was found to be 11 ± 0.57 mm at 100 mg/mL. The present study confirmed that the synthesis of ZnO-NPs calcinated at 500 oC using Rumex abyssinicus root extract is a promising approach for the development of efficient photocatalytic degradation of MG dye, and the ZnO-NPs calcinated at 300 oC provide antimicrobial efficacy against the four antibacterial strain.