Abstract: Objective: To investigate the changes of peripheral blood natural killer(NK) cell between inevitable abortion and successful pregnancy in patients with unexplained recurrent spontaneous abortion(URSA),and its clinical significance. Methods: Forty-four URSA abortion patients and 34 URSA pregnant patients were divided into the URSA abortion group and URSA pregnancy group,respectively.Forty-four normal pregnancy women and 44 normal non-pregnancy women were divided into the normal pregnancy group and normal non-pregnancy group(control group),respectively.The number of peripheral blood NK cells in four groups was detected using flow cytometry. Results: The ratio of NK cell to lymphocyte in URSA abortion group,URSA pregnancy group,normal pregnancy group and normal non-pregnancy group was(21.18±5.81)%,(16.12±4.13)%,(15.16±3.23)% and(14.91±3.18)%,respectively.The ratio of NK cell to lymphocyte in URSA abortion group was higher than that in URSA pregnancy group,normal pregnancy group and normal non-pregnancy group(P<0.01),the differences of which between URSA pregnancy group,normal pregnancy group and normal non-pregnancy group were not statistically significant(P>0.05). Conclusions: The occurrence of URSA may be associated with the enhancement of NK cells to embryo Rilling.The number of NK cells in peripheral blood can be used as an index in monitoring URSA patients.