The electrical conduction mechanism in La3 Ta0.5 Ga5.5 O14 (LTG) was studied from the standpoint of defect formation during growth caused by the nonstoichiometry of the crystal. Since the stoichiometric composition of LTG does not coincide with the congruent composition, single crystals grown from the stoichiometric melt under an atmosphere of O2 /Ar in the oxygen partial pressure range from 0.005 to 1 atm was Ta-poor. Ta vacancies were formed during the growth, and their population was predominated by the growth- p O2. With increasing temperature up to 600 °C, the Ta vacancies were ionized to yield holes, and thus the electrical conductivity of the LTG crystal varied with temperature and growth- p O2. In contrast, Ga vacancies as a carrier source would form by Ga evaporation during growth under p O2 below 0.005 atm. The growth- p O2 dependence of the electrical conductivity was opposite to the case for Ta vacancies. For the case of sintered LTG with the exact stoichiometric composition, impurity defects dominated electrical conduction below 600 °C, where the impurity concentration was higher than the cation vacancies arising from the nonstoichiometry of the sintered LTG. Thus, electrical conductivity was almost independent of the sintering atmosphere of p O2. Above 700 °C, the dominant carrier in both single crystal and sintered LTG was ions related to oxygen defects formed by the anion Frenkel reaction.