At the Experimental Farm of Ismailia Agriculture Research Station, Ismailia Governorate, Egypt, the experimental work was carried out over the course of two successive growing seasons in 2018–2019 and 2019–2020 in order to investigate the effects of iron oxide nanoparticles (7.5, 15.0, and 30.0 ppm) in comparison to unsprayed (control plants) on seed yield, growth and active Silybum marianum L. constituents. Three repetitions of a randomized complete blocks design were used for the experiment. Spraying Silybum marianum L. plants with Fe3O4-NPs at 30 ppm increased plant height, number of leaves/plant, number of inflorescences/plant, root length, stem diameter, dry weight of roots, number of roots/plant, inflorescences/plant and herb, chlorophyll a, chlorophyll b and total chlorophyll (a+b), N, P and K contents in leaves, seed yield /feddan and Fe concentrations in fruits, TXF, SBN, SCN, SDN, ISBN and total silymarin in fruits, in both seasons.