A low energy electron beam (EB) can let the self-interstitial atoms (SIA) cause the self-assembly (DSA), i.e., {311}SIA platelet in c-Si is discussed. We have studied how an SIA migrates toward metastable sites that form the platelet. We used a molecular dynamic (MD) simulation to trace all the atoms and crystallographic analysis method (PM) method to analyse crystalline defects. In MD, we skipped the EB irradiation stage that produces SIAs. Instead, before MD some Frenkel pairs (FP) or SIA coupled with a vacancy were randomly distributed in bulk. Then we monitored how contributed the surrounding target atoms to the DSA process. When the FP concentration is higher than 3 atomic percent (3 %), the number of atoms to be stayed at metastable sites looked to be saturated. However, it increased significantly at lower FP’s concentration, e.g., at 1 %. At the same time, somewhat cooperative and oscillatory movement of target atoms in bulk was found in terms of the long-rangeorder (LRO) parameter. The oscillatory change of LRO continued for a few tens of ps, which is much slower than the so-called lattice vibration. Therefore, we suppose the cooperative motion like slow phonon might promote the migration of SIA and increase the number of atoms on {311}SIA planes, whereas no such global wavy motion was observed in cases including much higher FPs, as if somewhat pinning effect had occurred therein.