Purpose: This study was designed to evaluate the healing process in horizontal alveolar ridge distraction of the narrow alveolus in dogs. Materials and Methods: Six beagle dogs weighing approximately 9 to 10 kg were used in this experiment. Horizontal alveolar ridge distraction was performed in the right mandible, where the premolars had been extracted 12 weeks previously. Twelve days after the completion of distraction, the lengthening apparatus was removed. The distracted site was evaluated at 12 and 24 weeks after the removal. Results: At 12 weeks, thin woven bone was observed at the distracted site growing from the surface of the original lingual cortical bone toward the transport segment. At 24 weeks, the distracted site had fully changed into new mature lamellar bone, but the transport segments had been almost completely resorbed. Discussion: Horizontal alveolar ridge distraction was performed successfully in beagle dogs even though the apparatus was removed 12 days after the completion of distraction. The most important feature of this technique is the resorption of the transport segment. Conclusion: Horizontal alveolar ridge distraction can be a beneficial technique for augmenting the alveolar ridge horizontally in the buccolingually reduced alveolar process before the placement of implants. Keywords: alveolar ridge augmentation, alveolar ridge distraction, distraction osteogenesis