The aim of this thesis was to investigate the biological effects of LIPUS on orthodontically induced inflammatory resorption (OIIRR) in in vivo and in vitro models. The in vitro samples were obtained from healthy human premolars and cultured for 5-days or 24-hours with application of LIPUS at different doses. The in vivo, we tested 10 beagle dogs where orthodontic movement was carried out for four weeks with a continuous force of 1 N/side;​ using a split mouth model. After 4-weeks, mandibles were resected into blocks involving the fourth premolar and its periodontal tissue. The 4th premolars were evaluated using micro-CT, histologically and immunohistochemically. In both models, LIPUS was generated with repetition rate of 1 KHz. Each pulse has a square envelop with duration of 200 microseconds and a carrier frequency of 1.5 MHz and 30mW/cm2 intensity. Daily application of LIPUS on TSOC for 5-days did not have any effect on the predentin thickness layer, and had an adverse effect on the odontoblast cell count. A one-time application of LIPUS in a 5-day culture of TSOC increased the predentin thickness in all groups. Also it increased the odontoblast cell count in the 5, 10 and 15 minutes application groups. LIPUS application for 10 minutes upregulated the expression of collagen-I and DMP-1 in the short term (24 h), where 5-minutes application upregulated the expression of collagen-I only. LIPUS did not affect the rate of orthodontic tooth movement and had a trend of increasing it with increased population of the osteoclasts attached to the alveolar bone in the PDL. LIPUS significantly reduced the number of OIIRR initiation areas by 71%, reduced its total volume by 68%, and reduced its volume relative to the affected root total volume by 70%. LIPUS induced the formation of precemntum layer, thicker cementum and reparative cellular cementum.