Abstract: The objective of this study was to investigate the effects of seasonal factors on in vivo embryo production and pregnancy rate after embryo transfer in Dorper sheep.In 2011,multiparous pure Dorper ewes from a breeding sheep farm were selected as donors and were treated with synchronized estrus,superovulation and laparoscopic artificial insemination.The embryos of pure Dorper ewes were obtained by surgical operation.Meanwhile,the receptor ewes were subjected to synchronized estrus and embryo transfer.Statistical analysis results showed that a total of 4 241 donors were subjected to the treatment of synchronized estrus in 2016 and 4 180 of them were received superovulation treatment;there were 4 131 and 3 987 ewes used as mating and embryo flushing donors,respectively,with a donor availability rate of 94.01%.A total of 23 516 flushed embryos were obtained,among them 18 002 ones were available,indicating that the embryo availability rate was 76.55%.The average number of available embryos produced by per donor was 4.52.The average number of flushed embryos and available embryos obtained from donors in October,November and December were significantly higher than those in April,May and June(P<0.05),and the highest average number of flushed embryos and available embryos was observed in October(P<0.01)which were 7.04 and 5.62,respectively.Some of the obtained embryos were used for embryo transfer.It was found that the number of single embryo pregnant recipients was 4 446,with the pregnancy rate of 53.76%;the number of double embryo pregnant recipients was 805,with the pregnancy rate of 65.02%.In addition,the conception rate of receptors was found higher in January,March,June and December(P<0.05)than the other months.This study provides a theoretical basis for superovulation and embryo transfer of Dorper breeding sheep and industrial production of mutton sheep.