Abstract: Ultrasonic-B was used for the diagnosis of early pregnancy in Rhesus monkey, 23 cases at 18-30 days of gestation were confirmed in 22 with one suspected,15 cases at 31-60 days of gestation were all confirmed by the diagnosis. In early pregnancy monkeys,a region of even and strong echogram and single gestational sac（GS） may be seen in their uteri. Most of the GS were round or oval in shape and the size increased with the length of the gestational period. With the advance of the gestation,fetus and heartthrob of the embryo may be seen. The size of the uterus in normal monkey during breeding season was less than 28mm in length and 22mm in width, no strong echogram region was found in the uterus.