The three-dimensional structures of carbon deposits on piston surfaces in a large-scale two-stroke marine diesel engine have been nondestructively investigated using synchrotron X-ray microtomography. The results indicate that the piston carbon deposit (PCD) has a flat bottom surface where many irregular structures accumulate. Based on the microtomography, cross sections of the carbon deposit sample can be obtained in the direction perpendicular to the piston surface. A uniform deposition is found on the bottom surface of piston, where many tiny pores appear, which implies that a liquid film first forms on the piston surface and subsequent nucleation boiling occurs due to the high surface temperature. A significant decrease in the PCD density is observed when the PCD thickness is below a threshold of approximately 150 mu m. However, the PCD density remains almost unchanged from 150 to 1800 mu m. This result suggests that there should be different dominant formation mechanisms at these two stages. The local morphology, especially the approximately circular structure and the smooth boundary on the PCD surface, indicates that condensation should be the major factor controlling the formation of the PCD surface. However, the internal pores and minimal swelling on the surface could result from other mechanisms. (C) 2014 Elsevier Ltd. All rights reserved.