We have prepared thin ordered NiO(100) films by evaporation of Ni in an O-2 atmosphere onto Ag(100). The films have been analyzed by using scanning tunneling microscopy and low-energy electron diffraction. For room temperature deposition a c(1 x 2) Ni/O Ag structure in two orthogonal domains is obtained in the submonolayer coverage range. Annealing such a film produces regular islands with NiO(100) double layers. For higher coverage (five monolayers) a layer-by-layer-like growth mode is obtained.