The relation between crystal structures and ferroelectric properties of x[BaBi2Ta2O9] - (1-x)[Bi3TiTaO9] solid solution was investigated through Rietveld analysis and polarization measurements. The single phase was confirmed in all composition range, and the orthorhombic to pseudo-tetragonal phase transformation was observed at x around 0.4-0.5. Curie temperature (T-C) decreased with an increase in x from 875 degreesC(x=0) to around 90 degreesC(x=1.0), and relaxer behavior characterized by the frequency dependence of T-max (temperature of dielectric permittivity maximum) was identified for the samples with x greater than or equal to 0.6. The occupation refined by the Rietveld analysis revealed that the cation disordering (about 15%) of Bi in Bi2O2 layers and Ba at the perovskite A-site was observed for BaBi2Ta2O9 (x=1.0). This cation disordering was suggested to play an important role in the relaxer behavior and ferroelectric properties.