Abstract: The changes of hemorheologic parmeters were observed during venovenous extracorporeal membrane oxygenation （ECMO） in six mongrel dogs. Normal body temperature maintanied and only slight hemodilution was used during the procedures. Blood viscosity, red blood cell aggregation and blood yield stress fell along with time in all dogs. However, red blood cell deformability was found to be increasing. This defect may result in microcirculation disorder. To miniminize the blood trauma, the quality of artificial devices need to be perfected. And cautions should be taken during the practical procedure.