By mixing NiCl2 and NH4HCO3 solutions at room temperature, NiO nano-particles were synthesized by hydrothermal modification and sintering. The agglomerated amorphous precipitates formed at room temperature were mainly composed of NiCO3. Ni(OH)2 and Ni(OH)2 crystallites and were converted to deposits with higher crystallinity and less content of Ni (OH)2 and H2O after hydrothermal treatment. NiO nano-particles prepared from the hydrothermal deposits were of higher dispersion than those from the amorphous precipitates. The primary thermodynamic calculation in Ni2+-NH3-CO32--H2O system indicated that the rise of temperature is favorable for the release of Ni2+ from the Ni2+-NH3 complex ions, accelerating the formation of NiCO3 and Ni (OH)2. The thermodynamic possibility for the formation of NiCO3 phase increased with the increase of temperature.