We characterized the growth behavior of nano NiO crystals synthesized by heat-treatment in air at the elevated temperatures using nickel nitrate ($Ni(NO_3)_2{cdot}6H_2O$). The crystals had the octahedral shape and the length of 200${sim}$500 nm. The truncation was observed when the NiO powder was heated up to $900^{circ}C$ for 2 hours. but not be seen at under $600^{circ}C$. It was observed that nano NiO crystals synthesized at $900^{circ}C$ made up the networks, the coalescence of the nano crystals through the neck formation between the crystals appeared as the first stage in the sintering mechanism.