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Nitrogen-doped anatase TiO2 nanoparticles were synthesized by a two-step wet chemical method. The N element is proved to be incorporated into the lattice of TiO2 with an average concentration of 3.3 at%. Magnetic measurements show that the undoped TiO2 is ferromagnetic at room-temperature (RT), which can be ascribed to a universal ferromagnetism (FM) in nanoparticles of metal oxides. However, significant enhancement of the FM was observed in N-doped TiO2 nanoparticles and it is robust against high-temperature annealing. Our results confirm the first-principle prediction that spin-polarized holes induced by N doping can mediate the spin alignment of the N dopants, leading to the strong RT FM in anatase TiO2 .