In this paper, we examined the morphology, shape and magnetic and electronic properties of Selenium-doped nickel oxide nanoparticles (Se-doped NiO-NPs), which were achieved through a sol-gel technique that involved the usage of Cydonia oblonga plant extract. The structural and magnetic properties of Se-doped NiO-NPs were evaluated by the employment of XRD, FESEM / EDAX, FT-IR, UV-Vis, and VSM procedures. According to XRD studies, the nanoparticles accommodated a face-centered cubic (fcc) crystalline structure and a space group of (Fm3m). In addition, the size of nanoparticles in optimal conditions (the optimum temperature of 400 °C and 3% Se-doped) were reported to be 7.7 nm while a direct relationship was also observed with increasing the concentration of selenium. The FESEM images confirmed the spherical morphology of Se-doped NiO-NPs. Also, the photocatalytic properties of Se-doped NiO-NPs were evaluated through the usage of methylene blue (MB) pigment degradation under UV light. The outcomes of this evaluation exhibited more than 76 % of degrading within 200 min. To complete the project, the cytotoxicity aspect was also investigated on L929 cell lines, requiring the application of MTT assay, while the results were indicative of toxicity effects that can be used for inhibiting cancer cells.