In the present work p-type Si specimens were implanted with Cl ions of 100keV to successively increasing fluences of 1×1015, 5×1015, 1×1016 and 5×1016ionscm−2 and subsequently annealed at 1073K for 30min. The microstructure was investigated with the transmission electron microscopy (TEM) in both the plane-view and the cross-sectional view.The implanted layer was amorphized after chlorine implantation even at the lowest ion fluence, while re-crystallization of the implanted layer occurs on subsequent annealing at 1073K. In the annealed specimens implanted above the lowest fluence three layers along depth with different microstructures were found, which include a shallow polycrystalline porous layer, a deeper single-crystalline layer containing high density of gas bubbles, a well separated deeper layer composed of dislocation loops in low density. With increasing ion fluence the thickness of the porous polycrystalline layer increases. It is indicated that chlorine can suppress the epitaxial re-crystallization of implanted silicon, when the implant fluence of Cl ions exceeds a certain level.