To determine hypoxia-inducible factor 1α (HIF-1α) and its target gene, vascular endothelial growth factor (VEGF) and receptor (VEGFR-1) concentrations in the placentas of the donor and recipient in monochorionic twin pregnancies with twin-twin transfusion syndrome (TTTS).Twenty monochorionic twin pregnancy cases were included in the study (10 with and 10 without TTTS). Tissue protein expressions of HIF-1α,VEGF and VEGFR-1 were determined by using immunohistochemistry. Western blot analysis were used to quantify and compare the protein expression. RT-PCR were used to compare their mRNA expressions.HIF-1α was mainly observed in trophoblastic cells and villi capillaries endothelial cells, and VEGF in trophoblastic cells, endothelial cells and villi stromal cells; VEGFR-1 was mainly observed in villi trophoblastic cells and vascular endothelial cells. The placenta protein and mRNA expression of HIF-1α and its target gene in the donor placenta increased significantly (P0.05). There was no difference between the controls.When the monochorionic twin placenta is formed in the early period, HIF-1α, VEGF and VEGFR-1 are over-expressed, which may affect the placenta angiogenesis and induce TTTS .