The fundamental mechanism of the hydrogen exfoliation phenomenon that occurs at the damaged layer in H ion-implanted silicon was investigated. A damaged layer formed by high-dose hydrogen implantation in a silicon wafer was observed by cross-sectional transmission electron microscopy (XTEM), and (1 0 0) platelets and (1 1 1) platelets were visible. Density and size of the platelets in the damaged layer were analyzed quantitatively. Although the density and size of (1 0 0) platelets are almost twice those of (1 1 1) platelets, the density of (1 1 1) platelets in the deeper area is greater than the density of (1 0 0) platelets. Neither the densities nor size of either (1 0 0) or (1 1 1) platelets have any relation with implantation dose in the dose range from 5.0 x 10(16) to 8.0 x 10(16) (H cm(-2)). The sizes of (1 1 1) platelets in the deeper area approach a certain size (about 10 nm), because of their partial combination with (1 0 0) platelets.