Interstitial (I) clustering in Si has been investigated using a hierarchy of atomistic approaches: the tight binding molecular dynamics (TBMD). the molecular dynamics (MD) based on environment dependent interatomic potentials (EDIP) and a lattice kinetics Monte Carlo (LKMC) code. The model implemented in the LKMC code reproduces the energetic derived by means of MD calculations. The modalities of elementary kinetic steps have been tested using EDIP. Kinetics, simulated by LKMC, shows a peculiar evolution pathway characterized by quite well defined phases. The nucleation phase is dominated by clusters containing few interstitials in a over coordination state. In dependence on thermodynamic constrain, droplets form which store Is in more adjacent compact clusters. Droplets formation favors Is organization in <1 1 0 > elongated chain. A qualitative comparison of simulation results with experimental data on post-implanted Si characterization is reported. (C) 2001 Elsevier Science B.V. All rights reserved.