Objective To assess the relationship between endometrium damage and focal distance of high intensity focused ultrasound ablation(HIFU).Methods 150 patients with requirement of atoke from Ⅱ type uterine fibroids under HIFU were collected and divided into two groups by the location of fibroids,one was antetheca group, and the other was posterior group,which were further divided into three groups,A group(25 cases),B group(25 cases), and C group(25 cases),in which their distance were 15mm,18mm,and 20mm from endometrium respectively.Those patients were treated with HIFU,and improvement of symptoms was assessed by uterine fibroids symptom (UFS) score,energy efficiency factor(EEF)was served as total energy required by treatment,the closest distance from mar-gin of necrosis of fibroids to endometrium and the rate of ablation were detected by MRI,as well as integrity of endo-metrium and vaginal bleeding were observed on postoperative 2 days and 3 months.Results After treatment by HIFU,significant improvement of symptoms was found,especially for the fibroids in antetheca(t =3.868,P =0.01), moreover,as for the fibroids in antetheca,the obvious efficacy was found in B group and C group(F =4.711,P <0.05),while for posterior fibroids,the efficacy was not associated with the location of HIFU(F =3.898,P >0.05).In addition,high rate of ablation and low EEF for the fibroids in antetheca was found(t =9.818,11.224,respectively,all P <0.01),and the highest rate of ablation in A group was revealed(F =105.673,P <0.01),followed by that of B group,the worse was C group.While in posterior fibroids,the distance was not associated with the rate of ablation(F =0.485,P >0.05).There was significant difference of integrity rate for endometrium between antetheca and posterior fibroids on postoperative 2 days(93.33% vs.77.33%,χ2 =7.67,P <0.05),and the rate in antetheca fibroids was higher than that of posterior fibroids(94.67% vs.82.67%)on postoperative 3 months.After postoperative 2 days,as for the fibroids in antetheca,the rate of integrity in B group(100.00%)and C group(100.00%)was higher than that in A group(80.00%),while in posterior fibroids,the highest rate was in C group(100.00%),followed by B group, which of A group was the worst(60.00%);After postoperative 3 months,in the fibroids of antetheca,there was no sig-nificant difference of rate between B group and C group,which were higher than that in A group(84.00%),while in posterior fibroids,the highest rate was in C group(100.00%),followed by B group(84.00%).In addition,significant difference between varied focal distance and improvement of vaginal bleeding existed in antetheca and posterior fibroids(P <0.05),for example the score of vaginal bleeding in B group and C group at postoperative 3 months was lower than that in A group(F =7.292,P <0.01),while for posterior fibroids,the efficacy of C group was higher than that of B group(F =14.559,P <0.05 ).Conclusion Although improved efficacy of Ⅱ type uterine fibroids is offered by HIFU,for the minimum damage of endometrium,its focal distance is varied with the locations of fibroids, namely,the safe distance in antetheca is more than 18mm,while that in posterior fibroids is more than 20mm.