Abstract: In order to investigate the changes of ultrasonic characteristics of the embryos of jennies, two jennies were examined with B-ultrasonic rectal linear probe（6.5 MHz） for the whole gestation. Based on the obtained B-ultrasonic examination results, the pregnancy period of the jennies was divided into four stages: stage 1（14～＜60 d）, stage 2（60～＜100 d）, stage 3（100～＜190 d）, and stage 4（190～350 d）. The results showed that at stage 1, pregnancy could be determined by B-ultrasound scanning of the uterus, the width of the uterus was about 26～110 mm, the lateral uterine angle was significantly enlarged, round liquid dark areas and strong echo bands appearing in the solid dark areacould distinguish early embryos, umbilical cords, allantoic cords, etc.At stage 2, the uterus would become larger and larger, part of which moved into the abdominal cavity, the fetus was located at the bottom of the uterus, the fetal images, were hardly obtained by B-ultrasound, showing up occationally, and fetal fluid without ultrasonic reflection could be observed.At stage 3, the fetal organs such as ribs, head, eyes, spine and other obvious features could be observed by B-ultrasound. At stage 4, the fetus would enter the pelvic cavity, and the fetal head and forelimbs could be detected through rectal examination and B-ultrasound.