First-principles calculations with projector augmented wave method has been carried out for Sr- and/or Mg-doped rhombohedral LaGaO3. The fort-nation energies of 10 kinds of defects have been systematically computed in consideration of temperature and oxygen partial pressure. Sr2+ ion is found to be stable at La site rather than Ga site, whereas Mg2+ ion is stable at Ga site. The solution of dopants accompanied with the formation of oxygen vacancies shows the lowest energy at temperatures of 500-1600 K and oxygen partial pressure of 10(-21) to 1 atm. Defects resulting in n- or p-type conductivity are energetically unfavorable under these conditions. The present theoretical results are consistent with experimental results of doped LaGaO3 in the literature regarding both site preference of dopants and the dependence of ionic conductivity on oxygen partial pressure. Substituting Sr2+ and Mg2+ ions with the formation of O2- vacancies are confirmed to be predominant defects in temperatures of 500-1600 K and oxygen partial pressures of 10(-21) to 1 atm in r-LaGaO3 by means of first principles calculation.