The morphology of ultra nanocrystalline diamond (UNCD) films is modeled by a pure geometrical approach. Diamond spheres with a fixed radius are added randomly spread on a smooth substrate surface covered with diamond seeds in varied primary nucleation density. The secondary nucleation rate and the nucleation density on the bare substrate define the distribution of diamond spheres during each cycle. In preliminary results, this simple geometrical model leads to good comparison with published experimental data; low primary nucleation densities yield to the growth of separated diamond islands with a rough surface, the smoothest films are obtained with the highest primary nucleation densities.