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Platelet adhesion is one of the most pivotal events of blood clotting for artificial surfaces. However, the mechanisms of surface-induced platelet activation have not been fully been elucidated or visualized so far. In this study, we attempted to observe the internal structures and adhesion interfaces of human platelets attached to artificial surfaces by transmission electron microscopy (TEM) during the platelet activation process. We prepared observation samples by a conventional embedding method using EPON 812 resin. The sectioning was sliced perpendicular to the a-platelet/material interface. Observation by TEM indicates that internal granules coalesce in the center of the platelet accompanied by pseudopodial growth in the early stage of platelet activation. Pseudopodia from a platelet attach to the material interface not along a plane but at a point. In addition, along with the process of platelet activation, the gap between the platelet membrane and the material surface at the interface disappeared and a-platelet/material adhesion became much tighter. In the fully activated platelet stage, the platelet becomes thinner and tightly adheres to the substrate. As a result of comparative observation of an adherent platelet on polycarbonate (PC) and on amorphous carbon (a-C:H), it was found that internal granules release was inhibited more remarkably on a-C:H coating rather than on PC. Despite numerous technical difficulties in preparing sectional samples, such a study might prove the essential mechanism of biomaterial-related thrombosis, and it might become possible to modify the surfaces of materials to minimize material-related thrombosis.