A new material, manganese-doped holmium zirconium oxide solid solution, was synthesized in a reducing atmosphere at 1623 K. Microstructural characterization of the material was performed using transmission electron microscopy (TEM) in conjunction with energy-dispersive X-ray spectroscopy (EDXS). It was confirmed that the material consisted of two phases having a similar basic cell related to the fluorite-type structure. One of the phases builds up the matrix and reveals the fluorite-type structure. The other phase has a structural modulation along the <110> direction of the basic cell, with spacing two times that of the (110) plane, and occurs as small precipitates, 70-100 nm in size, embedded in the matrix. A lattice correspondence was maintained between the precipitates and the matrix with a small strain. The holmium content in the precipitates is higher than that of the matrix, which gives rise to the structure modulation along the <110> direction.