Fe3O4@C supported ultrasmall and highly monodisperse ruthenium (Ru) nanoparticles (Ru/Fe3O4@C) were prepared as novel nanocatalysts for hydrogenation reactions. The catalysts displayed high activity for the hydrogenation of rosin to form hydrogenated rosin with extraordinary selectivity. The role of controlled carbon size on the catalytic properties of the catalysts was studied. The Ru/Fe3O4@C catalyst can be easily separated from the organic solvent by applying an external magnetic field and the separated Ru/Fe3O4@C can simply and efficiently used in ten consecutive runs without significant decrease in activity. Significantly, Ru/Fe3O4@C nanocatalysts showed a wide substrate scope for hydrogenation reactions under mild conditions. [GRAPHICS] .