To produce bioactive peptides from the by-products of fish processing, bigeye tuna (Thunnus obesus) head was hydrolyzed by alcalase, and the tuna head protein hydrolysate (THPH) was fractionated through ultrafiltration membranes with a range of molecular weight cutoffs (MWCO) of 8, 5, 3 and 1 ku, respectively, to yield the fraction THPH-1 with MW distribution<1 ku. The antioxidant activities of THPH-1were evaluated using reducing power, free radical scavenging and aged model mice induced by D-galactose. The physicochemical properties of THPH-1 were analysed also. THPH-1 showed evident radical scavenging activity in a dose-dependent manner with the IC₅₀ values for hydroxyl radical, superoxide radical and 1,1-diphenyl-2-pycrylhydrazyl (DPPH) radical being 1.38, 0.73 and 0.93 mg/mL respectively. The reducing power of THPH-1 was 0.763 at 12.5 mg /mL. The activities of superoxide dismutase (SOD) of liver and the activities of glutathione peroxidase (GSH-PX) of liver and serum in aged model mice were significantly increased (P<0.05) compared with that of the normal control group mice after 42 days administrated orally at a dose of 30 mg/kg THPH-1, and the content of malondialdehyde (MDA) of serum in aged model mice was significantly decreased (P<0.01).The physicochemical properties showed that THPH-1contained 96.40% of protein(on dry basis), 0.11% of fat and 4.86% of ash. The total hydrophobic amino acids content of THPH-1was 35.8% of the total amino acids content. The molecular weight distribution of the higher activity fractions of THPH-1were located at 1 802-2 519 u and 422-922 u.These results indicate that THPH-1from tuna head hydrolysate would be very fairly helpful in the preparation of antioxidative peptides and a beneficial ingredient for functional food.