Electronic structures and vacancy formation of Li3N doped with 3d transition metals were studied using first-principles density functional theory simulations. It is found that dopant ionic radius may determine its substitution site in Li3N. Ti energetically favors substituting trigonal planar Li atoms (Li(2)) while other 3d transition metals prefer substituting Li atoms between layers (Li(1)). V, Cr, Mn, Fe, Co and Ni substitutions significantly lower the energy band gap with localized electrons at the Fermi level, suggesting induced electronic conduction. Transition metal substitution generally reduces Li vacancy formation energy and hence enhances the Li vacancy concentration. It is revealed that transition metals except Co and Cu strongly trap the neighboring Li vacancies based on the calculated dopant-vacancy binding energies, and may retard the Li ionic conduction. Our calculations confirm the reported excellent performance of Co-doped Li3N as anode material with improved ionic and electronic conduction, and suggest the relation of improved electronic conduction with localized electrons as well as the relation of enhanced ionic conduction with decreased vacancy formation energy and insignificant dopant-vacancy binding energy.