A medical X-ray computed tomography (CT) scanner was applied to dynamic measurements of beach profile changes, internal sediment structures, and sediment transport processes during wave actions in the laboratory experiments. An acrylic flume 7 m long and with an inner rectangular cross-section of 0.3 m × 0.3 m passes through the mobile gantry of a medical X-ray CT-scanner. Reconstructing three-dimensional images using Hounsfield Units (HU), the relationships between beach profile changes and sediment density distributions within the depth of disturbance (DOD) are examined. The DOD is located within 9 mm below the bottom surface and sixty times larger than mean sand diameter (d50). A negative correlation is found to be existed between them.