Monodisperse　nickel　oxide　(NiO)　nanoparticles　with　tunable　size　have　been　synthesized　by　hot-injection　method.　The　NiO　nanoparticles　were　characterized　by　X-ray　diffraction,　transmission　electron　microscopy,　ultraviolet　and　visible　spectrophotometer.　It　was　found　that　the　amount　of　oleic　acid　controlled　the　phase　structure　of　the　nanoparticles.　The　reaction　temperature　has　a　significant　effect　on　particle　size　and　optical　properties.　The　synthesized　NiO　nanoparticles　(5-100　nm　in　diameter)　have　good　uniformity　and　monodispersity.　A　reasonable　formation　process　of　the　NiO　nanoparticles　was　proposed.　(C)　2016　Elsevier　B.V.　All　rights