Background and Objective: Obesity, a public health problem is growing in prevalence over the past decade. Obesity increases mortality risk and there is evidence that obesity in youth is a more powerful predictor of this risk than obesity in adulthood. The purpose of the present study is to examine the effect of walking exercise in order to reduce obesity in sedentary obese girls. Materials and Methods: This research was carried out on 20 non-athlete volunteer obese girls (BMI>30) between 19-25 years and then were randomly divided in two groups (Control: n=10 and Experimental: n=10). At the beginning and after 2 months, fat mass, percent body fat and lean mass were assessed with bioelectrical impedance equipment (BIA) and serum insulin measured by enzyme-linked immunoassay (ELISA) analysis. Then the experimental group started to do exercise program which consisted of 30 minutes walking with intensity of %50- %75 of maximal heart rate, 3 sessions in a week for 2 months. The data was analyzed using t-test. Results: The results of this study showed that walking exercise , positively reduced body fat percent and fat mass and increased lean mass in exercise group (p= 0.000). Serum insulin also significantly decreased (p = 0.008). Conclusion: This study demonstrated that a regular physical activity program such as walking exercise can significantly reduce body weight and improve serum insulin in obese girls. In conclusion, it seems that this type of training can be efficient, safe and inexpensive way in order to reduce and prevent obesity.