ABSTRACT The present project work was mainly focused on reviewed structural and optical properties of Nickel oxide (NiO) nanoparticles. I calculated the band gap and particle size of NiO nano particles from UV-Vis and PL spectra using brus model of effective mass approximation for different temperatures and solvents effect. The UV-Vis spectra shown that as temperature increased from 240 to 280 ℃ the energy band gap decreased from 4.1 to 3.61 eV, however the particle size increased from 5 to 75 nm. For different solvents energy band gap increased from 3.74 to 3.99eV, when particle size decreased from 9.6 to 5.82 nm. At PL spectra of peaks wavelengths (303, 332, 364 nm) of different solution temperatures 37, 50, 70 ℃, particle size and energy band gap are determined. As temperature increased from 37 to 70 ℃ the particle size increased from 3.38 to 7.68 nm, but band gap energy decreased from 4.09 to 3.4 eV. And also when NiO is doped with barium the particle size decreased from 3.6 to 0.42 nm, whereas the energy band gap increased from 3.3 to 6.15eV. In my work by increasing temperature at both UV-Vis and PL spectra the calculated value of the energy band gap by brus mass approximation is increased, but the particle size of NiO nanoparticles is decreased. For increasing the solvent the energy band gap decreased, however; particle size is increased. Key words: NiO, nanoparticles, X-ray diffraction, UV-Vis spectroscopy, PL spectroscopy, Brus mass approximation, Energy band gap, particle size.