The objective of this experiment is to observe structural differences in the orthodontic movement of vital (control group) and devitalized (experimental group) teeth in the mongrel dogs. The 5 utilized dogs in this experiment were approximately 1 year of age and their average weight was about 12Kg. Endodontic therapy was performed on the 2nd premolars in upper & lower jaws of each animal under the general anesthesia by intravascular injection of 25mg/Kg of pentobarbital sodium. The canals of the teeth were obturated by using gutta percha in conjunction with root canal sealer (AH26 Densply). One of the roots in the 2nd premolars was hemisected to make an extraction space for the devitalized teeth to be moved. The edgewise technique was employed for the movement of the teeth. Orthodontic models and intraoral roentgenograms were taken before and after orthodontic tooth movement. The open coil springs (.010 x .040) were used at interbraket space in order to provide equal forces (75gm) between the teeth in each arch wire. After 13 weeks of active orthodontic tooth movement, dogs were sacrified and the experimental results were examined through the intraoral radiography, microscopic examination and scanning electromicroscopic examination at the root surface. From the results of the study, the following conclusions may be drawn: 1. The root resorption and cemental deposition were observed within the pressure and tension site in both group. 2. The root resorbed lacunae were observed in the cementum and/or into the dentin in both group. 3. The prominent osteoblastic activities were observed on the alveolar margin in the tension site in both group. 4. A few of blood vessels were observed in the pressure site, but also lots of blood vessels were observed in the tension site especially in the periphery of the alveolar bone in both group. 5. In the pressure site, resorbed lacunae were formed with deep and narrow cavity in the control group; the shallow and wide cavity in the experimental group. 6. In the pressure site, the repaired cementum or cementoid tissue was lined on cementum in the experimental group, but not in the control group. 7. There was no significant difference between external root resorption of endodontically and vital teeth when both were subjected to orthodontic forces.