On the basis of first-principles calculations. we present a structural model for the formation of H-induced (111) platelets in Si, which involves a structural transformation from a double-layer-H-2(*) configuration of HT aggregates into an H-saturated internal (111) surface structure. This reaction process preferably occurs at high H plasma treatment temperatures and subsequently generates H-2 molecules in the platelet voids, consistent with experiments. Our model also reveals the important features observed in(111) platelets, such as high-resolution transmission electron microscopy images, step structures. lattice dilation lengths, and H vibrational frequencies.