$La_{1/3}NbO_3$ and $La_{1/3}TaO_3$ large single crystal was prepared by the infrared floating zone method and their crystal structure and dielectric properties were investigated from the viewpoint of A - site vacancy. In $La_{1/3}TaO_3$ crystal, the crystal symmetry and approximate lattice constants was tetragonal and a $approx$ 0.397, b $approx$ 0.397 and c $approx$ 0.775 from the nonextinction and the reciprocal lattice point length seen on Weissenberg and Precession photographs. The jump of some $2/3La^{3+}$ ions leads to increase the moving paths due to the generation of new vacancies at middle empty sites and thus this result is confirmed to provoke the dielectric anomaly.