Objective:To investigate the effects of body mass index(BMI)on speed of sound(SOS)of the tibia in young and postmenopausal women.Methods:SOS were measured by bone quantitative ultrasound(QUS)apparatus in399young and589postmenopausal women,who were devided into three groups(low body weight ,normal weight and overweight )according to their BMI.Result:There was no significant difference in SOS among three groups(F=0.2,P=0.801)and no significant correlation between BMI and SOS in young women(r=0.054,P=0.287).Signifi-cant positive correlation was found between BMI and SOS in postmenopausal women(r=0.115,P=0.006),and the SOS of overweight group was significantly higher than that of the low body weight group and normal weight group(F=4.2,P=0.016).The incidence of osteoporosis in overweight group and normal weight group was signifi-cantly lower than in low body weight group (P<0.001).Conclusion:BMI has no significant effect on tibia strength in young women,but higher BMI can reduce the rate of loss of the tibia strength in postmenopausal women.