The dielectric losses of Si2N2O, studied theoretically and experimentally up to 1573 K at 10 GHz, demonstrated an increase above 1000 K due to the impurity conduction losses, because of the small Li activation energy. Calculations based on the first-principles theory were performed to obtain the density of states. The optical and elastic properties of Si2N2O doped with a 4.8 % molar fraction of Li were modeled using a plane-wave pseudopotential method to investigate the effect of Li incorporation into Si2N2O. The polycrystalline static dielectric constants of Si2N2O increased after it was doped with Li. We also provided the optical properties of silicon nitride compounds to be used as references. The calculated elastic constants of pure and Li-doped Si2N2O indicated that the elastic modulus of the Li-doped Si2N2O was smaller than that of the pure Si2N2O. Additionally, Li-doped Si2N2O crystals were more brittle than the pure forms. Theoretical analyses of the dielectric losses and first-principles calculations emphasized that to consider Li-doped Si2N2O materials for applications, the dielectric losses and Li contents should be carefully optimized, since these factors also affect the conductivities of these materials.