The Na_0.5_Bi_0.5_TiO_3-BaTIO_3_ system, at certain composition range, is a new kind of uItrasonic transducer ceramics material which has good and unique properties. By means of X-ray powder diffraction method, we precisely determined the cell parameters of Na_0.5_Bi_0.5_TiO_3, and propose a new type of Iattice structure for it different from that in previous reports. For this system, the Iattice Parameters at different compositions were determined. And the preliminary discussions on the relations between the properties, cell dimensions and compositions as well as the phase relation were also given.