Gene flow was analyzed among four sheep breeds of Mongolian Group in China using structural gene loci and microsatellite markers.Results showed that genetic differentiation was 0.0164-0.0455 by using structural loci, and was 0.0107-0.0239 by using microsatellite loci.This illustrated that most variations existed within breeds and genetic differentiation level was very low among sheep breeds of Mongolian Group in China.There was substantial Gene flow among the breeds as estimated by the structural loci (Nm=7.971) or the microsatellite method(Nm=15.732).There was no direct relationship between genetic differences from breeds and geographical distances.It is concluded that genetic differentiation of sheep breeds of Mongolian Group in China is mainly the result of natural selection (different living conditions).