In microcirculation, RBC deformability is one of the most important factor. In this study, the effect of aldose reductase inhibitor (ARI) on RBC deformability in diabetic rats was investigated. Seven-week-old Wistar male rats were used as a diabetic model. We injected streptozotocin (STZ) (55mg/kg) intraperitoneally to make them diabetic. And then we administered ARI orally to the STZ rats. We classified them into STZ injected diabetic group (DM) and STZ and ARI treated diabetic group (ARI). Experiments were performed three weeks after STZ treatment. The RBC deformability was measured directly from the microscope images of mesentery. From this experiment, it was found that ARI didn't alter HbA1c, but improved the decreased RBC deformability in diabetes to the normal level.