In this work we have investigated the average composition of Boron Interstitials Clusters (BICs) and the strain induced in the Si crystal by BICs. We have formed BICs by Si implantation and subsequent annealing of two Si samples, grown by molecular beam epitaxy, containing thin buried layers doped with different B concentrations (1019 and 1020at/cm3). By B chemical profiles diffusion analysis, we have extracted the doses of Si self-interstitials (I) and Boron atoms trapped at the BICs. The B/I stoichiometric ratio is about 1 for the low B concentration and about 3.5 for the high B concentration sample. High-resolution x-ray diffraction analyses provided an estimate of strain profile. While in the low B concentration sample no appreciable strain was detected after BIC formation, at the higher B concentration we found that the tensile strain present in the as grown B doped layer changes to a strong compressive strain as a consequence of BICs formation. For this kind of clusters, the mean volume expansion with respect to the Si matrix is of (29 ± 6) Å3 for each B atom trapped at the BICs.