We report the lattice strain and polarization of the BaTiO(3)-CaTiO(3) solid solution. We found that the lattice strain evaluated by the tetragonality of the tetragonal phase at room temperature is nearly independent of the composition within the limit of the solid solution. In association with this variation, the saturation polarization remains nearly unchanged. Such invariant lattice strain associated with the ionic displacement in ferroelectrics is considered to be responsible for the nearly compositional independence of the polarization and the observed ferroelectric Curie temperature. Its relatively stable polarization compared with that of pure BaTiO(3) is very interesting for technological applications, such as in ferroelectric memory.