Present work is aimed to obtain a clear idea of the phase stabilities of the title oxide at temperatures from 10K to 900K. X-ray powder and single-crystal diffraction measurements and neutron high-resolution powder diffraction measurements were carried out for the samples annealed in several different atmospheres i.e. CO + CO2 or O2 + Ar mixed gas, air, vacuum at about 10(-3)Pa. Clear evidence that NiO6 octahedrons rotate in the bc-plane (b > c > a setting) in La2NiO4.00 at room temperature was obtained. Shape of a magnetic peak 011 shows some broadening at temperatures below 70K i.e. LTO2 range.