The sintering and electrical characteristics of La-modified Na1/2Bi1/2TiO3 (NBT) was investigated from a defect structure viewpoint. To reveal the role of cation vacancies, two series of ceramics, with different cation vacancies, were processed to compensate the excess positive charge of lanthanum ions. In a region of complete solid solution, the grain size of NBLT-B {[(Na0.5Bi0.5)(1-x)La-x]Ti1-0.25xO3} was smaller than that of NBLT-A {[(Na0.5Bi0.5)(1-1.5x)La-x]TiO3} and densification was enhanced more effectively in NBLT-B. With the aid of thermoelectric power, electric conductivity, and electrotransport measurements, it was found that different sintering behaviors between NBLT-A and NBLT-B specimens were related to the change in the type of cation vacancies present and that lanthanum ion-cation vacancy pairs played an important role in reducing the grain growth and enhancing the densification process.