Aim To investigate role of the morphologic and hemodynamic changes of contralateral vertebral artery(VA) in the occurrence of posterior circulation ischemia(PCI) in patients with unilateral vertebral artery occlusion using color Doppler flow imaging(CDFI) combined with transcranial Doppler(TCD). Methods 96 patients with unilateral vertebral artery occlusion were recruited into this study.Among them,there were 50 cases with PCI and 46 cases without PCI.Diameters of contralateral VA were measured by CDFI.Peak systolic velocity(PSV) and end diastolic velocity(EDV) of contralateral VA and basilar artery(BA) were measured by TCD. Results The presence rate of collateral anastomose on DSA had no differences between two groups(P>0.05).The diameters of contralateral VA in patients without PCI group were 3.54±0.47 mm,which was significantly larger than that in patients with PCI group(3.25±0.45 mm,P><0.01).The PSV and EDV of intracranial segment in contralateral VA in patients without PCI was dramatically higher than VA with PCI(87.09±35.47 cm/s and 35.85±18.03 cm/s vs 70.60±31.04 cm/s and 27.32±11.75 cm/s,P><0.05).The PSV and EDV of BA in patients without PCI group was higher than that with PCI group(89.54±35.56 cm/s and 37.35±19.34 cm/s vs 72.98±25.95 cm/s and 29.52±11.56 cm/s,P><0.05). Conclusion In patients with unilateral VA occlusion,the hemodynamic changes of contralateral VA and BA was strictly related to the occurrence of PCI.