Background: Ramadan fasting is an obligation for many Muslims around the world who abstain from eating and drinking for one month, which has different medical and physiological effects, such as reducing blood pressure, lipid profile, blood glucose, and body weight. It has also been hypothesized that Ramadan fasting may induce some changes in the hematologic parameters. Thus, we aimed to investigate the effect of Ramadan fasting on blood cell count (CBC), and erythrocyte sedimentation rate (ESR). Methods: In the present study, 59 adult healthy individuals, who had completed one month of Ramadan fasting were included. Fasting blood samples were analyzed for ESR, hemoglobin (Hb), hematocrit (Hct), white blood cell (WBC), platelet count (PLT), mean corpuscular Volume (MCV) and mean platelet volume (MPV), one day before, on the second and last week of Ramadan and one month after Ramadan (phase I , II, III, and IV, respectively). Results: 34 men and 25 women with an age range of 15 to 24 years participated in the study. Mean ESR increased significantly (except phase IV, in comparison phase III). Mean Hb and Hct levels were significantly greater in phase III than phase I (PConclusion: Ramadan fasting has statistically significant effects on hematological parameters, which were within normal range