The aim of the study was to evaluate the beneficial and preventive effects of quercetin on some hematological parameters in streptozotocin (STZ)-induced diabetic rats. Experimental animals were divided into four equal groups as Control (C), Diabetes (D), Quercetin (Q) and Diabetes+ Quercetin (DQ). STZ was injected at a single dose of 60 mg/kg (ip) for diabetes induction. Quercetin (15 mg/kg) was injected (ip) to the Q and DQ groups (after diabetes had happened) during 4 week. In blood samples, leukocyte (WBC), erythrocyte (RBC), hemoglobin (HGB), hematocrit (HCT), platelet (PLT), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC) and differential leucocyte count were examined. In the study RBC and platelet counts, Hb and Htc levels in diabetic rats significantly decreased compared with control (C) and quercetin (Q) groups. These parameters were tending to increase in DQ group compared with D group. MCV, MCH, MCHC levels did not show any important changes in all groups. Leucocytes count was increased with diabetes and decreased obviously with quercetin treatment. Neutrophil count was increased in diabetic rats. On the other hand lymphocyte count was decreased in D group. And there were no significantly differences neutrophil and lymphocyte count with quercetin treatment in DQ group. In conclusion our results showed that further studies are needed about quercetin and its effects on diabetes with different amount and duration.