The regional differentiation of chromosomes and haemoglobin of Tan Sheep, a special fur breed in dry semi-desert in the Northwest of China, were studied with the ecogeographical points of view and methods, and the interesting results had been discovered. The fur charac-terestics, the chromosomal aberrations, the types of haemoglobin (Hb), and the other economic performances of the populations distributed in the desert, desert-steppe and dry steppe regions had been examined. The same items were also examined in the population of Mongolian Sheep, the ancestor of Tan Sheep, but is distributed in Inner-Mongolia Plateau. The regularity of geographical differentiation is concerned with the same differentiation of natural environments. Five parts of this paper are: 1) the main characteristics of the populations of Tan Sheep; 2) the ecological environments of those populations; 3) the regional differentiations of chromosomes; 4) the regional differentiations of haemoglobin (Hb); and 5) conclusions.