Objective To investigate the ultrasound imaging findings of anatomical relationship between femoral artery and vein in children of different ages.Methods Sixty-five children aged 4 months-7 years were enrolled in this study.The children were divided into 3 age groups: group Ⅰ＜ 1 yr;group Ⅱ 1-3 yr and group Ⅲ＞ 3,≤ 7 yr.A protable ultrasound machine was used.The probe was placed at the level of inguinal ligament and 2 and 4 cm below inguinal ligament.The children were placed in supine position.The legs were placed in 2 positions:(1)extended and in standard anatomical position and(2)flexed and 45° abducted and 45° laterally rotated.Results The examination showed that at the level of inguinal ligament,the femoral vein lay behind and lateral to femoral artery in 91% of children.At the level of 4 cm below inguinal ligament,the femoral vein lay posterior and lateral to the femoral artery in all children.When the leg was placed in abducted and laterally rotated,the depth of femoral vein was reduced and the vein was less overlapped by artery in all children,especially in preschool children.Conclusion At the level of 4 cm below inguinal ligament,the femoral vein lies posterior and lateral to the femoral artery in children.When the leg is placed in abducted and laterally rotated,the depth of femoral vein is reduced and the vein is less overlapped by artery.It is indicated that femoral vein puncture should be performed at the level of 4 cm below inguinal ligament with the leg flexed and abducted in all children,especially in preschool children.