The purpose of this study was to find out the effects of a 12-week aerobic exercise plus lifestyle modification on obese-induced metabolic syndrome in obese adolescent girls. A total of 52 obese adolescent girls (13-14 years old; body mass index (BMI) ${geq}$95th percentiles for age and sex) purposely assigned to aerobic exercise group (AEG, n=15), aerobic exercise plus lifestyle modification group (ALG, n=18), or control group (CG, n=19). The AEG completed 12 weeks of walking exercise (30-60 min/day, 65-75% HRmax, 6 days/week), the ALG completed 12 weeks of walking exercise (30-60 min/day, 65-75% HRmax, 6 days/week) and behavior modification (60 min/day, 1 day/week), and the CG continued their normal life. The presence of the metabolic syndrome and component risk factors were determined before and after 12-week programs. The total prevalence of the metabolic syndrome was 48.1% in this sample (25/52) of participants at baseline. After the programs the prevalence of the metabolic syndrome was improved in the AEG and ALG 33.3, 27.8%, respectively. Group analyses showed significant difference in risk factors of the metabolic syndrome such that the AEG and ALG had significantly greater improvements in waist circumference, triglycerides, blood glucose and systolic blood pressure than the CG, while there were no significant difference in HDL cholesterol and diastolic blood pressure. Also there was no group difference between AEG and ALG in all measured metabolic risk factors after the programs. These results indicate that the positive changes of the ALG were not associated with lifestyle modification (behavior modification) but associated with aerobic exercise. However, long-term follow up studies are necessary to clarify the additive effect of the behavior modification on the metabolic syndrome.