Thiazolidinediones (TZDs) induce insulin sensitization through the activation of PPAR${gamma}$. However, the undesirable effect such as weight gain was observed. The purpose of this study was to find out an herbal drug that could reduce the side effects of pioglitazone. Among herbal formula that we have searched, oryung-san (OR) inhibited the differentiation of preadipocytes and did not affect on glucose uptake in 3T3-L1 adipocytes. In vitro, glucose uptake assay and Oil Red-O staining in 3T3-L1 adipocytes were conducted. In vivo, pioglitazone (PIO, 30 mg/kg), oryung-san (OR, 300 mg/kg), or pioglitazone co-administered with oryung-san (PIO+OR) were administered orally for 7 weeks in high fat diet (HFD) fed ICR mice and measured the body weight and blood glucose level every week. PIO+OR group significantly reduced body weight gain, triglyceride, and total cholesterol compared to PIO group. In addition, PIO+OR group showed a significant reduction of plasma glucose level (72%) compared to HFD control group. Insulin levels in PIO+OR group was also markedly decreased by 85% and 41% compared to HFD control and PIO group, respectively. Diameter of white adipocytes was decreased in the PIO+OR group compared to that in PIO group. Moreover, PIO+OR group reduced expression of PPAR${gamma}$ and SREBP1a compared to PIO group. Taken together, oryung-san can improve side effects of pioglitazone, such as weight gain and edema, and shows a synergistic effect in plasma insulin levels.