Objective To study the expression and the mechanism of miR-155in the villi of patients with unexplained recurrent spontaneous abortion (URSA).Methods The expression of miR-155 in the villi of 36 cases with URSA (URSA group) and 25 women with normal early pregnancy (control group) were detected by stem-loop real-time reverse transcription (RT) qPCR.Expression of hypoxia inducible factor-1 (HIF-1α),vascular endothelial cell growth factor(VEGF) and micro lymphatic vessel density (MVD) in the villi of were measured by immnohistochemical staining among two groups.Results (1) miR-155 expression:the mean miR-155 expression were 1.456 (0.489,2.459) in URSA group and 2.833 (1.740,3.794) in control group,which reached statistical difference (P ＜0.05).The mean expression of miR-155 of 1.683 (0.902,2.459) in URSA group with abortion times (≤ 3) was significantly higher than 1.229 (0.489,1.719) in URSA group with more than 4 times abortion (P ＜ 0.05).(2) Indexes:the expression of HIF-1α,VEGF and MVD value were 121 ± 12,134 ± 12,36 ± 6 in URSA group and 99 ± 10,109 ± 10,28 ±4 in control group,which reached statistical difference(P ＜ 0.01).The expression of HIF-1α,VEGF and MVD value of 119 ± 12,134 ± 12,35 ± 5 in URSA group with less than 3 times abortion was significantly lower than 128 ± 12,138 ± 12,43 ± 6 in URSA group with more than 4 times abortion (P ＜ 0.01).Conclusions The expression of miR-155 and HIF-1α is topically stimulated by oxygen signal.HIF-1α adjusts the transcription and translation of VEGF,which together involved in placental trophoblast invasion and placental angiogenesis.The low expression of miR-155 could interfere with expression of HIF-1α and VEGF,which might be involved in villous vascular dysplasia in URSA.