The systems ZrO2-Ln(2)O(3) have been studied on samples annealed at 600, 1170, 1450 degrees C in the 0-15 mol % Ln(2)O(3) (where Ln is the rare-earth La, Nd, Sm or Er) range using X-ray diffraction, thermal analysis and dilatometry. The microstructure of annealed samples was examined mainly by electron microscopy. It was found that rare-earth oxides-doped zirconia formed monoclinic, tetragonal, cubic and pyrochlore-type phases. The existing region of the tetragonal phase is 1-15 mol % Ln(2)O(3), which is independent of the species but dependent on the dopant content and temperature. The equilibrium phase diagrams and non-equilibrium diagrams have been deduced. The temperature and composition of eutectoid ZrO2,SS (T)-->ZrO2,SS (M) + Py, as well as interconnection between grain size, Ln(2)O(3) content and the martensitic transformation temperature, (M(s)), were determined.