To evaluate the mechanical deformation phenomena of dental abutment screw after tightening and loosening, five species of abutments were selected (3i-three, Avana- two) respectively by two pieces. The abutment screws were repeatedly tightened and removed 20 times with a digital controller. Field emission scanning electron microscope (FE-SEM) was used to observe changes of each part caused by repeatedly tightening and loosening experiment. As the number of tightening and loosening was increased, the wear or distortion of hexed or squared slot that contacted with the driver tip was more severely progressed. Wear or distortion of hexed slot was more severe than that of squared slot and it was more remarkable in the titanium screw than in the gold screw. All the tested screws showed that the width in the crest of their screw thread decreased gradually as the test was proceeded.