Introduction. Blood group diet is a health concern in obese subjects. Therefore, this study aimed to investigate the effect of high intensity interval training (HIIT) and blood group (O) diet on weight reduction in obese subjects. Methods. Sixty class II obese subjects between 40 and 45 years old with blood group (O) were randomly assigned into 2 equal groups. Group (A) included 30 subjects received blood group O diet and HIIT 3 sessions /week for 12 weeks. Group (B) included 30 subjects received caloric diet and HIIT 3 sessions /week for 12 weeks. Before and after the study, measurements including body weight, body mass index (BMI), waist-to- hip ratio (WHR), body fat percentage (BF%), basal metabolic rate (BMR), glycated hemoglobin (HbA1c), serum uric acid, creatinine and blood urea were done.