Collagen, in a form suitable for human consumption, can be obtained from the large amounts of fish skin via protein hydrolysis to produce low-molecular-weight substances with enhanced bioavailability and function. To optimize this method, we investigated the synergistic effect of elevated temperature (150-250 degrees C) and pressure (350-3900 kPa) of a hydrothermal process on the hydrolytic ability and characteristics of collagen hydrolysates. Elevated temperature and pressure of the hydrothermal process increased the levels of free amino groups and lower-molecular-weight collagen hydrolysates, particularly at 210 degrees C and 2100 kPa. The resulting hydrolysates were fractionated by ultrafiltration membranes of different molecular weight cutoff and evaluated for their antioxidant (ARTS radical scavenging activity and reducing power) and anti-aging (tyrosinase and collagenase inhibition) activities. The < 1 kDa fraction had the highest antioxidant activities, whereas the 5-10 kDa fraction had the highest anti-aging activities. Therefore, fish skin could be successfully modified into biologically active collagen peptides by a hydrothermal process (hydrolysis) and ultrafiltration (separation), and the resulting bioactive peptides have potential for development as antioxidants and anti-aging ingredients in the food, nutraceutical, and cosmetic industries.