Evidence on the possible mechanisms for the use of Omega 3 fatty acids to mediate obesity requires clinical studies continue with specific methodologies. The aim was to assess the effect of omega-3 supplementation on Body Mass Index (BMI), Wais - Hip Index (WHI) and body composition of obese women using bioelectrical impedance. Subjects 60 premenopausal obese women (BMI > 30Kg/m2) were randomly assigned to 3 groups: Group 1) placebo, vitamin E (200 IU), group 2) 1 g of omega and group 3) 2 g of omega-3. All of them received a low calorie diet and moderate exercise. Weight, BMI, WHI, and fat distribution were measured at the beginning and every month for three months. The results show us Omega-3 supplementation significantly reduced weight, BMI, and total fat mass, compared to the control group, a dose-response effect. These effects depended on the time and amount of Omega 3 supplemented, when the degree of compliance of exercise, adherence to the diet and age were controlled. In conclusion the supplementation with omega-3 is an efficient method in the management of obesity in premenopausal women.