Thermal expansion behaviors of Ba(6-3x)Ln(8+2x)Ti(18)O(54) (Ln=La, Nd, and Sm, x=0.5, 0.67, and 0.75) ceramics were determined by dilatometric measurement. The samples of all investigated compositions expanded nearly linearly with increasing temperature in the range of 20 degrees-1200 degrees C. Their thermal expansion coefficients were determined to be 10.7-11.4 ppm/degrees C. A discontinuous change in sample size was observed at about 1350 degrees C for each composition, indicating the existence of a phase transition. As determined by the high-resolution X-ray diffraction analysis, the phase constitution and the lattice parameters of Ba6-3xSm8+2xTi18O54 ceramics were maintained in the quenched samples. The origin of the phase transition was discussed thoroughly. Phase equilibrium in the BaTiO3-Ln(2/3)TiO(3) system was reviewed by considering the phase transition.