The amount of wear on posterior composite resin was studied using the 3-D measurement milling apparatus(type A-1).An extracted sound premolar was selected and a class I cavity was prepared on the occlusal surface.The cavity was filled with posterior composite resin and finished(base-line).The base-line impression was taken with silicone rubber impression material.Artificial wear was applied by sand-blaster with a glass-bead.Then, the second impression was taken as a post-worn specimen.Both impressions were electroplated by copper and the dental stone was poured into the electroplated impressions to make the measuring dies.Two measuring dies were digitized with A-1 and the data were registered onto the disk.Both data were denoted on the computer CRT display or X-Y plotter, and compared with the scanning electron microscopic photographs.The measuring accuracy of the A-1 system was 5μm in X, Y and Z directions, and it could be increased up to 2μm.The wear could be observed clearly and measured easily in the two-dimensional display.The A-1 system could display the data at any location, any magnification and desired section.One of the remaining problems is the measuring speed.However, this could be solved by a modification of the software program including the vector movement.