Abstract The reduction of nickel in silica gels was measured as a function of nickel content within the temperature range 600–900°C. This leads to the formation of metallic nickel nanoparticles homogeneously dispersed in this amorphous matrix. Furthermore, a Transmission Electron Microscopy (TEM) study has revealed that the size of these particles is distributed following a log-normal law. Moreover, this study has shown that the particles size is nearly independent of the nickel content while its distribution becomes wider when it increases. Magnetic measurements have shown, whatever nickel concentration and treatment temperature are, a superparamagnetic behavior. Finally, magnetization simulations, as a function of applied magnetic field, have been performed considering Langevin function in order to calculate a theoretical particles-size distribution and to compare it with the experimental one.