Abstract： Objective To compare the effect of bone regeneration with two different ratios of nano-hydroxyapatite(nHA)/collagen(Col)(nHAC) after tooth extraction in canine.Methods Two kinds of nHAC grafts were prepared with different nHA/collagen ratio of 3 ∶ 7 and 5 ∶ 5.Eighteen male healthy adult dogs had been randomly divided into three groups.Immediately after extraction of the mandibular second premolars,each kind of nHAC was implanted into extraction sockets as follow:Group A,nHA/Col=3∶7(12 sites);Group B,nHA/Col=5∶5(12 sites);Group C,blank control group(12 sites).The bone repairing abilities of the two grafts such as vertical distance of alveolar,CT values,general observation,histological observation,trabecular volume fraction and porosity were separately analyzed at 1st,3rd and 6th month,respectively (each group had 4 sites in different time periods).Results nHAC were absorbed gradually after they were implanted into alveolar bone defect and were replaced by new bone.The vertical distance of alveolar bone in Group A([15.76±0.28] mm) was significant higher than that in Group B([14.88±0.36] mm),and CT values of Group A([879±31] HU) were higher than those in Group B([718±29] HU)(P＜0.05).The trabecular bone volume percentage of Group A([22.2±0.4]％) was higher than that in Group B([20.3±0.4]％),and the bone porosity of Group A([23.6±0.9]％) was lower than that in Group B([27.6±0.6]％)(P＜0.05).In addition,the vertical distance,CT values and the trabecular bone volume percentage of Group C was lower than those in Group A and B,but the bone porosity of Group C was higher.Conclusions The nHAC with nHA/ collagen ratio of 3:7 could better promote bone regeneration than nHAC with the nHA/collagen ratio of 5 ∶ 5 did.