Hydrothermally synthesized powders of bismuth sodium titanate, (BiNa)Ti2O6 (BNT) and bismuth sodium titanate doped with 1 and 2 at% lanthanum, respectively were sintered under atmospheric pressure at 1 000°C for 1...3 hours. Measurement of the dielectric permittivity of the La-doped samples showed a broad maximum spanning the Curie temperature that decreases with increasing frequency (dielectric relaxation). The dielectric loss tangent (tan δ) was found to decrease steeply with increasing frequency. This is in accordance with the typical frequency-dependent behavior of a ferroelectric relaxor ceramics whose disordered complex perovskite structure shows diffuse order-disorder (OD) phase transition. Sintering of undoped BNT resulted in strongly decreased dielectric permittivities and increased dielectric losses compared to La-doped BNT. This can be related to thermal decomposition of BNT owing to evaporation losses of Na2O and Bi2O3.