Background:Medicinal plants have a long history in treating blood disorders, which is one of the most common problems in today's advanced world. Fennel (Foeniculum vulgare) is a medicinal plant with a high content of polyphenols and has antioxidant and immunomodulatory properties. Objectives: The aim of this study was to evaluate the effect of hydro-alcoholic extract of fennel on some hematological indices in male rats. Materials and Methods: In this experimental study, thirty male Wistar rats were divided into six groups (five rats in each group). The first group (control) did not receive any dose; the second group (sham) received 1 mL normal saline (extraction solvent); and the experimental groups 1, 2, 3 and 4 respectively received 1 mL hydro alcoholic extract of fennel in four doses of 250, 500, 750 and 1000 mg/kg of body weight every 48 hours for 30 days by gavage. One day after the last gavage following induction of anesthesia and taking blood from the heart of rats, measurement of red and white blood cells, hemoglobin, hematocrit and tests of bleeding and coagulation time (CT) were performed. The data was analyzed by one-way ANOVA test using SPSS15 software. Results: Fennel increased mean RBC (7.54 ± 0.53 × 106) and WBC (5.89 ± 0.78 × 103) values, especially at a dose of 250 mg/mL and CT (2.45 ± 0.20) at a dose of 500mg/mL compared to the control group (P < 0.05). Conclusions: Fennel increased red and white blood cells probably due to the presence of polyphenols and antioxidant activity of fennel and reduced negative effects of free radicals on blood cells.