The effect of La substitution for Sm was investigated in Ba6-3x(Sm1-yLay)(6+2x)Ti18O54 (x=0.5) ceramics in the entire range of y (0 similar to 1.0). X-ray diffraction (XRD) analysis indicated that the phase constitution of the present ceramics was sensitive to y, and an abnormal phase transition from the tungsten bronze-type to a new Ba16Ce32Ti64O192-type solid solution was determined at y=0.6-0.9. The crystal structure of the new phase was refined by Rietveld refinement. The stability of the Ba16Ce32Ti64O192-type structure was discussed with the ordering of cations and the average radius of Ln(3+). The Ba16Ce32Ti64O192-type phase was observed only in the sintered pellets but not in the loose powders in the investigated temperature range of 1200 degrees-1350 degrees C. A differential scanning calorimetric analysis (DSC) was performed for investigating the thermal effect of the phase transition. Discontinuous dielectric properties were observed corresponding to the phase transition. The Ba16Ce32Ti64O192-type phase has a much higher dielectric constant, a lower quality factor, and a larger positive temperature coefficient of resonant frequency at microwave frequencies in comparison with the tungsten bronze-type phase. Besides, the dielectric loss of the Ba16Ce32Ti64O192-type phase is much lower (tan delta similar to 10(-4)) at frequencies below 10(8) Hz, while the dielectric constant is insensitive to the frequency.