The geometrical structure of Mg,Zn,Cd doped 32-atom supercell of InN was optimized using the ultra-soft pseudopotential method of total-energy plane wave based upon the density functional theory (DFT). Cell parameters of both doped and undoped cells were calculated theoretically. The binding energy, partial density of states, Mulliken charges, electron density differences of doped InN crystals were calculated and discussed in detail. The results revealed that compared with Zn and Cd, Mg substituting for In has the greatest solubility and yield more states of holes.So Mg is suitable for p-type doping of InN.