To investigate the effect of swimming on some hematological parameters in diabetic rats is the main goal of this study. 32 adult healthy rats were divided into Control (C), Diabetic (D), Exercise Control (E) and Diabetic+Exercise (DE) groups respectively. The rats in the control group were not soever treated, while the D and DE groups were injected 60 mg / kg streptozotocin (STZ) intraperitoneal (i.p). The trial continued for four weeks after the onset of diabetes. After the diabetes was formed, 1-hour swimming exercise for 4 weeks, 5 days a week was applied to groups E and DE during the trial period. At the end of the experiment (after 4 week) some hematological parameters (such as RBCs, Hb, Htc, platelet, WBC, differential leucocyte count) were determined in blood samples. RBCs, Hb and Htc values were reduced in D group. On the other hand, WBC and platelet count were increased in the same group. We observed RBC count, Hb and Htc levels were higher and WBC count was lower in DE group according to D group. As a result; current study was observed that regular aerobic exercise improves investigated parameters and demonstrates positive effects.