Abstract: This study was aimed to explore the underlying reason of low implantation rate after embryo transplantation. Exogenous hormones and CIDR were used to treat Small Tail Han sheep, the estrogen receptor （ESR） gene expression in ovarian cells derived from estrus synchronization treatment (EST)，natural estrus control (NEC) and anestrus (ANE) Small Tail Han sheep were compared, and the methylation levels of CpG island was analyzed. The results showed that there was no significantly difference in estrus performance and time between EST and NET sheep, and the transcript levels of ESR were also not significant difference (P>0.05), but they were significantly higher than that in ANE sheep ovarian (P<0.05). Furthermore, though the expression amount of ESR at protein level was greatly higher in EST sheep ovarian compared to that ANE sheep (P<0.05), it was significantly lower than that in NEC sheep (P<0.05). Methylation levels at the CpG island of ESR promoter was 0 and 10.77% in NEC and EST sheep, respectively; while the promoter was hypermethylation in ANE sheep（92.30%）.These results suggested that EST sheep showed similar estrus performance and time to that of NEC sheep after treated with exogenous hormones, but ESR gene expression at protein level was significantly different，and the methylation level of promoter was also significant difference. Perhaps it was one of the main causes resulting in the low implantation rate after embryo transplantation of Small Tail Han sheep.

Key words: ESR gene; Small Tail Han sheep; estrus synchronization; gene expression