Abstract: This research was designed to detect the association between single nucleotide polymorphisms (SNPs) of ActRIIB gene and vertebra number variation in Small Tail Han sheep (Ovis aries). Using SNP analysis techniques, specific primers were designed to amplify the sequences of ActRIIB gene of the Small Tail Han sheep, PCR products from sheep of different thoracolumbar spine amount phenotypes were sequenced. A C85T mutation was found in the amplified fragment which caused MspⅠrestriction site disappear. Three genotypes, AA, AB and BB were produced after digestion by restriction enzyme MspⅠ, of which BB was mutation homozygote. The result of Chisquare test showed that genotype frequency was significantly different among different vertebral patterning sheep (P<0.05). The homozygous mutation was only found in normal T13L6 and T14L5 individuals, its frequency was highest in individuals with T14L5 phenotype. Thus it could be inferred that the mutation may give rise to the first lumbar vertebra changing into the fourteenth thoracic vertebra. At the same time, the effect of the point mutation in the 4th intron′s on the vertebra development of Small Tail Han sheep was found.