We investigate the electronic structures of n-type doped CuInS2 crystals with chalcopyrite structures using Zn or Cd species, Cu-substituting species, as donor dopants based on ab initio electronic band structure calculations. We find that the strongly localized impurity states for n-type CuInS2 doped with Zn with small decrease in Madelung energy compared with that for n-type CuInS2 crystals doped with Cd species. This finding indicates that a deep donor level below the conduction band will be formed for n-type Zn-doped CuInS2 crystals, resulting in high-resistivity n-type CuInS2:Zn. This is in good agreement with other experiments. The total energy calculations show that the formation of Cu vacancy in the vicinity of Zn sites, donor-impurity sites, is energetically favorable for Zn-doped CuInS,. In other words, the Zn-donor states are compensated by the Cu-vacancies generated by the Zn doping. On the other hand, for n-type Cd-doped CuInS2 crystals, we find more distant Cu vacancies from the Cd sites. From the above findings concerning both the electronic structures and compensation mechanism of Zn-or Cd-doped CuInS, crystals, we predict that Cd species can be considered as suitable candidates for use as donor dopants, to fabricate a "buried-homo junction, p-CuInS2/n-CuInS2:Cd/n-CdS" of CuInS2-based solar cells.