Lanthanum nickelates with Ruddlesden-Popper structure (La2NiO4, La3Ni2O7, and La4Ni3O10) and perovskite structure (LaNiO3) have attracted considerable attention due to their potential applications such as solid oxide fuel cells. Currently, the ionic and electronic conduction properties of La3Ni2O7, La4Ni3O10, and LaNiO3 are not fully understood because it is quite difficult to prepare their dense bodies required for the characterization. The difficulty arises from their narrow thermodynamic stable temperature and oxygen partial pressure ranges. In this study, we successfully obtained dense bodies of single-phase La3Ni2O7, La4Ni3O10, and LaNiO3 via a post-sintering oxidation process. First, dense pellets composed of fine-grain precursors La2NiO4 and NiO (~0.5 mu m) were prepared by nitrate freeze-drying technique and low-temperature sintering at 1150 degrees C-1225 degrees C. Then they were converted into almost single-phase La3Ni2O7, La4Ni3O10, and LaNiO3 by high-temperature oxidation. La3Ni2O7 and La(4)Ni(3)O(10 )were obtained under an oxygen partial pressure pO2 of 1 bar at 1275 degrees C and 1200 degrees C-1250 degrees C, respectively, while LaNiO3 was obtained under pO2 of 392 bar at 1250 degrees C using hot isostatic pressing. The relative densities of the pellets exceeded 90%. With regard to their phase stability, decomposition was not detected at 600 degrees C-1100 degrees C in air for at least 100 hour despite their thermodynamic instability.