Abstract： Objective To investigate the effect of the continuous light force to the donor teeth on the periodontal healing after transplantation.Methods Thirty-two maxillary and mandibular incisors in four 10-month-old male Beagle dogs were autotransplanted.The pulps were removed in all teeth.The teeth were divided into four groups,one control and three experimental groups.In control group ( group 1 ),the teeth were unloaded.In the other three experimental groups,continuous force (0.49 N) was applied in the 1st (group 2),2nd (group 3 ) and 4th (group 4) week,respectively.The dogs were sacrificed in the 8th week.The tissue blocks were demineralized and sectioned perpendicular to the long axis of the teeth.The histological analysis was made. Results Histomophometric analysis revealed a significantly lower occurrence of replacement root resorption in the group 3 (2.1％ ) than in the control group( 12.5％,P ＜0.05 ).The significant lower incidence of replacement root resorption,and a higher surface and inflammatory root resorption were found in group 2(6.3％ and 68.8％ ) than in the control group( 12.5％ and 41.7％,P ＜0.05).No significant difference was found between group 4 and control group(P ＞0.05 ).Conclusions The orthodontic force promoted the regeneration of the periodontal ligament and prevented dentoalveolar ankylosis,whereas excessive initial force might cause root and bone resorption.