Silicon self-interstitial defects have long been a subject of great interest because of their scientific and technological importance. Single Si interstitials are highly mobile even at room temperature. Hence, in bulk Si most Si interstitials are likely to remain in the form of self-interstitial defects or interstitial-impurity complexes. The formation and structure of rod-like {311} defects have been well characterized by high-resolution transmission electron microscopy. In addition, a series of recent spectroscopy measurements have evidenced existence of small self-interstitial clusters (below a few nanometers in equivalent diameter) before they evolve into larger extended defects. In ultrashallow junction formation with low-energy implanted dopants, such small interstitial clusters are thought to be a main source for free interstitials responsible for dopant transient enhanced diffusion and electrical deactivation during post-implantation thermal treatment. Hence, significant experimental and theoretical efforts have been made to determine the structure and stability of small self-interstitial clusters as well as their growth and structural transition to larger extended defects, yet still unclear. In this presentation, we will focus on addressing the evolution of Si self-interstitial clusters from compact to rod-like extended structures. Using a combination of continuous random network model based Metropolis Monte Carlo, tight-binding molecular dynamics and density functional theory calculations, we have determined the structure and formation energies of n-interstitial clusters (n = 1 - 16). The extensive combined calculations demonstrate a distinct structural transition from the compact to elongated chain-like shape at n = 11 – 12. In particular, the I12 structure elongated along the [110] direction turns out to be very stable with a formation energy of 1.65 eV per interstitial, substantially lower than 2.79 eV, 2.06 eV and 1.85 eV as predicted for I2, I3, and I4 clusters. Using tight binding molecular dynamics simulations we have also found that additional interstitials placed around the I12 cluster preferentially migrates to the (110) cluster edges to grow larger. In addition, the predicted formation energies in general decreases with cluster size, however the variation exhibits an oscillating trend, with strong minima when n = 4, 8 and 12 due to the high thermal stability of the I4, I8 and I12 clusters. Our results are overall in good agreement with earlier inverse model studies based on experimental observations. We will also discuss structural reconfiguration of small compact clusters (n < 10) and evolution of rod-like defects with a {311} habit plane from small clusters.