The purpose of the present study was to investigate the presence, location and severity of root resorption after orthodontic buccal tipping movement using two magnitudes of force and register root repair following two different periods of retention. Fixed orthodontic appliances were inserted to the first premolars of twenty four patients having maxillary crowding and seeking orthodontic treatment. In each patient, force of 20g was applied to the right side and force of 50g.was applied to the left side. Active tooth movement lasted for 4weeks and followed by retention periods 4 & 8 weeks for each magnitude of force. All the specimens were examined with scanning electron microscopy. Three distinct types of resorption defects were detected: small isolated lacunae, wide shallow resorption bays, and deep lacunae. The number of each type of resroption defect and the distribution over the length of the roots varied with the magnitude of force. The pattern of repair was partial, functional or anatomical depending on the type of resorption defect and the retention period. In conclusion, the force of 20g resulted in few and shallow resorption lacunae mainly at the cervical and middle thirds of the buccal surface, while the force 50g resulted in deeper lacunae that were detected on the buccal surface with the middle and apical thirds of the lingual surface. Repair process started during active tooth movement and the healing potential increased greatly in the retention periods.