Lanthanum modified lead zirconate titanate (PLZT) ceramics are useful for electrooptic devices. However, as far as the defect structure of PLZT, there are still some arguments. In this study, the single phase widths of the completely pore-free transparent PLZT x/65/35 ceramics have been studied as a function of La concentration x. The PbO loss data corresponding to the upper and lower limits of the single phase from the starting composition were measured by the thermogravimetric method in controlling atmospheres. Also, the density approach corresponding to the single phase region has been tried for the almost completely porefree specimens prepared by Two-Stage Processing. The defect structures are discussed so as to fit in both the single phase regions. Some electrical properties are measured for specimens with various PbO deficiency within the single phase region.