This work reports on the lattice strain induced by small B clusters in Si produced by He irradiation of an epitaxial layer uniformly doped with substitutional B. By means of high resolution X-ray diffraction measurements and nuclear reaction analysis, lattice strain profiles and B lattice location of the sample before and after He irradiation were extracted, respectively. We found that, as a consequence of He irradiation, all B atoms form B–B pairs, and the tensile strain status of the as-grown B-doped layer lowers by a factor of about 2. Moreover, the volume lattice expansion ΔV per clustered B atom was found to be (−(10.0 ± 0.6) Å3). This negative value of ΔV is very different from the positive one (+(3.7 ± 0.6) Å3) found for B clusters created as a consequence of the B interaction with self-interstitials produced by Si implantation and subsequent annealing [G. Bisognin, D. De Salvador, E. Napolitani, A. Carnera, E. Bruno, S …