First-principles calculations have been conducted to investigate impurities in cubic Si4N4 and Ge3N4. Impurity species suitable for n-and p-type doping are suggested, in terms of the formation and ionization energies. The suggested species are P and 0 as it-type dopants and Al as a p-type dopant for c-Si3N4, and Sb and 0 as n-type dopants and Al as a p-type dopant for c-Ge3N4. The dependence of the formation energies on the chemical potentials indicates that a proper choice of growth conditions is mandatory for suppressing the incorporation of these impurities into anti and interstitial sites, where the impurities can be charged to compensate carriers.