Effects of niobium doping on dielectric and ferroelectric properties of unmodified lead zirconate titanate (PZT) and chromium modified lead zirconate titanate (Cr-modified PZT) have been investigated. The x-ray structural study shows the phase formed depending on the dopant concentrations with increasing the doping content tending to an increase in the rhombohedral phase in both of PZT and Cr-modified PZT ceramics. Furthermore, the Curie temperature (T _c ) and dielectric constant are decreased with increasing Nb concentration and the dielectric loss of Cr-modified PZT ceramics is lower than that of the PZT ceramics. From the hysteresis loop measurements, by increasing Nb concentration, the unmodified PZT ceramics show increase in the coercive field, while the remnant polarization decreases slightly. However, coercive field decreases, and the remanent polarization tends to increase with increasing niobium concentration from 1-3 mol% in Cr-modified PZT ceramics. Further increase in niobium concentration results in a drop of the remanent polarization and the coercive field.