Nitrogen molecular ion implantations have been performed in niobium thin films to form B 1-NbN layers with an accelerating voltage of 150 kV up to a dose of 5 × 1017 N2+ ions/cm2 at room temperature. Measurements of superconducting transition temperature (Tc), Auger electron spectroscopy analyses (AES), and x-ray diffraction analyses (XRD) have been carried out as a function of nitrogen dose in order to characterize the implanted layer. It has been found that there are two regions in the dose dependence of Tc; in the low-dose case, Tc decreases from the initial value of 8 K to less than 4.2 K with increasing dose, and in case of the high dose (more than 2.5 × 1017 ions/cm2) it increases rapidly to 11 K and becomes independent of the implantation dose. The XRD patterns indicate the generation of lattice disorder at the low dose, the formation of Nb2N and ∊NbN at the intermediate dose, and the formation of δNbN (B 1 structure) at the high dose; the selective growth of δNbN occurs at the highest dose, in spite of the fact that thermodynamically Nb2N and ∊NbN are equilibrium phases at room temperature and they are formed at the intermediate dose. In conclusion, high-dose nitrogen implantation at room temperature causes the formation of δNbN to increase Tc.