Bi7Ti4NbO21, Bi4Ti3O 12 and Nb-doped Bi4Ti3O12 la yer-structure ferroelectric ceramics were prepared. Microstructure, dielectric and piezoelectric properties of Bi7Ti4NbO21 were investigated by comparing with the electrical properties of Bi4Ti3O12 and Nb-doped Bi4Ti3O12. High resolution transmission electron microsco py result of Bi7Ti4NbO21 showed an intergrowth structure of perovskite -like(Bi2Ti3O10)2-, (BiTiNbO7)2- layers and (Bi 2O2)2+ layer along c direction. Two dielectric anomalies of Bi7Ti4NbO 21 occurred at 668 and 845℃. The material underwent a ferroelectric-ferroelectric and a ferroe lctric-paraelectric phase transition at the low and high temperatures. They were attributed to the microstructure transition of (Bi2Ti3O10)2- an d (BiTiNbO7)2- perovskite layers, respectively. During the depolarizat ion, the piezoelectric coefficient d33 of Bi7Ti 4NbO21 decrease d by 10% of its initial value over 200℃. The reason was that the directional ar rangement of defect dipoles created by some vacancies was damaged. The results o f depolarization experiment and thermally stimulated current measurement indicat ed that the piezoelectric properties of Bi7Ti4NbO 21 came from the polarization of intrinsic dipoles and defect dipoles.