Abstract: ObjectiveThe relationship of T/C mutation of MTNR1A gene ExonⅡand seasonal reproduction traits of sheep(Ovis aries) was investigated, in order to further research mechanism of MTNR1A gene in reproduction. MethodAltay sheeps and Chinese Merino sheeps, which are seasonal estrous sheep breeds, Hu sheep and Small-tail Han sheep, which are perennial estrous sheep breeds, were chosen to detect distribution of T/C mutation of MTNR1A gene ExonⅡ in these four sheep breeds by PCR-SSCP and sequencing. ResultThe result showed that, T/C mutation of MTNR1A gene ExonⅡ could cause change of random coil conformation of MTNR1A protein. T/C mutation was detected in four sheep breeds, and TT was main genotype in Altay sheep and Small-tail Han sheep, allele frequencies of T were 3.4 and 11.2 times as those of C, respectively. But in Chinese Merino sheep population, TC was main genotype, its genotype frequency was as 17.6 and 19.6 times as those of TT and CC respectively. There was non-significant difference in Hu sheep popula-tion. T/C mutational site of MTNR1A gene ExonⅡ was in Hardy-Weinberg balance status in Altay sheep population(P>0.05), but was in extreme imbalance status in Chinese Merino sheep, Hu sheep and Small-tail Han sheep populations (P<0.01). ConclusionT/C mutation of MTNR1A gene ExonⅡ can be detected in Altay sheep and Chinese Merino sheep which are seasonal estrous sheep breeds, as well as Hu sheep and Small Tail Han sheep population which are perennial e-strous sheep breeds, but this mutation is not correlated with seasonal reproduction trait of sheep.