Obesity can be defined as a metabolic disease due to an increased fat accumulation in the body caused by an imbalance of calorie intake and output The prevalence of obesity has increased substantially over the past 2-3 decades in developed and developing countries. The health impact of weight gain is so marked that obesity has now been classified as a major global public health problem In order to investigate the effect of diet conversion and oral administration of Platycodon grandiflorum extracts on the treatment of obesity, male Spraque-Dawley rats were divided into four groups: a group converted to normal diet (Control group), a group maintained high fat (30%) diet (H), and two groups with Platycodon grandiflorum extract added to the previously mentioned two groups. All animals were fed high fat diet for 7 weeks to induce the obesity. Then they were divided as mentioned above. Animals were fed experimental diet and Platycodon grandiflorum extract (150 mg/ml/rat/day) for 7 weeks. Body weight, adipose tissue weight (subcutaneous, epididymal, peritoneal fat pads) and serum lipids (total cholesterol and triglyceride) showed some differences among groups. The Platycodon grandiflorum feeding markedly decreased both body weight and adipose tissue weight in control group compared to H, high fat diet maintaining, group. Platycodon grandiflorum extracts significantly decreased the concentrations of serum lipids compared to H group. Fat cell numbers and sizes were significantly reduced in the oriental medicinal herb extract administrated group. Increased fatty acid binding protein (FABP) expression in high fat diet group was decreased by the dietary conversion to normal diet and the oral administration of Platycodon glandiflorum extracts. In contrast, there was no significant effect on FABP expression in the high fat maintenance group. In this study, the conversion from high fat diet to low fat or normal diet had a beneficial effect on body weight loss and serum lipid profiles. Dietary Platycodon glandiflorum extracts had an additive beneficial effect on the prevention and treatment of obesity.