The effect of A-site vacancy order-disorder states on diffuse phase transition (DPT) of tungsten bronze Pb1−xBaxNb2O6 (PBN) ferroelectrics has been investigated. The A-site vacancy disordered PBN ceramics exhibit notable variations of the Curie temperatures (i.e., the temperatures at the dielectric maximum permittivities) up to 12 °C in response to frequency changed from 0.1 KHz to 100 KHz. The largest frequency dispersion occurred at the morphotropic phase boundary of 1 − X = 0.63, along with the lowest Curie point. In contrast, the A-site vacancy ordered PBN ceramics present little frequency dispersion in the range of 2 °C from 0.1 KHz to 100 KHz. Dielectric constants of the disordered PBN ceramics were generally higher than those of the ordered ones. In comparison with the ordered PBN ceramics, the Curie points of the disordered ceramics were shifted from lower to higher temperature as the Pb2+ cation percentage was decreased; i.e., the Curie temperatures of the disordered PBN ceramics were lower than those of the ordered ones when 1 − X ≥ 0.70, but higher when 1 − X ≤ 0.65. These differences are suggested to inherently result from the A-site vacancy order-disorder states. The relationship between the A-site vacancy order-disorder states and the dielectric properties has also been confirmed with studies of thermal hysteresis.