Hydrothermal treatment of rice hull was hydrothermal carried out at 105, 110, $121^{circ}C$ for 15, 30, 60 min, respectively, using a conventional autoclave. Antioxidant activity of the hydrothermal treated rice hull extract was evaluated by determining total phenol contents (TPC), 1,1-diphenyl-2-picylhydrazyl (DPPH) radical scavenging activity (RSA), reducing power, and ABTS RSA. TPC, DPPH RSA, reducing power, and 2,2-azinobis(3-ethylenebenzothiozoline-6-sulfonic acid) (ABTS) RSA of the extract were significantly increased with increasing treated temperature and time. For example, hydrothermal extracts at $121^{circ}C$ for 60 min increased the TPC, DPPH RSA, reducing power, and ABTS RSA to 0.840 mg/mL, 64.77%, 1.437, and 92.11%, respectively, while those of the extracts treated at $105^{circ}C$ for 60 min were 0.508 mg/mL, 51.23%, 0.819, and 45.22%, respectively. The results indicated that hydrothermal treatment of rice hull was very effective to increase phenolic compounds and antioxidant activity of rice hull extract.