We report the elaboration of Ni doped ZnO nanoparticles prepared by a sot-gel processing technique. In our approach the water for hydrolyse was slowly released by esterification of the metal acetate with methanol followed by a supercritical drying in ethyl alcohol. Doping concentrations between 5 and 25 at% have been investigated. In the as-prepared state the powders with an average particle size of 30nm present ferromagnetic properties; thermal annealing in the 500 degrees C to 700 degrees C temperature range in air or oxygen modifies the magnetic properties. We ascribe the observed ferromagnetism to the presence and transformation of Ni based secondary phases.