A fixed fraction (20mol%) of PbTiO 3 was introduced into Pb[(Mg,Zn) 1 / 3 Nb 2 / 3 ]O 3 with systematic substitutions of Nb by Ta. Powders were prepared by two-stage reactions with repeated calcination to improve perovskite formation. Developed structures in the calcined powders were determined by X-ray diffraction. The low-frequency dielectric constant and loss values of the ceramics were measured. Dielectric constant spectra were further analyzed to investigate the diffuseness modes. Internal microstructures were examined using a scanning electron microscope.