Abstract: To examine the effects and physiological mechanisms of ultrasound treatment on the germination and seedling growth of naturally aged seeds, an [L₉(3⁴)] orthogonal matrix experiment was carried out, which included ultrasound time, ultrasound temperature, output power and seed soaking time with tall fescue Festuca arundinacea stored for one year and five years at room temperature, perennial ryegrass (Lolium perenne, stored for five years), and Russian wildrye (Psathyrostachys juncea, stored for six years) as test materials. The results showed that ultrasound temperature was the most important factor for the germination of aged seeds. The appropriate ultrasound treatment promoted germination percentage and significantly enhanced root and shoot length in aged seeds of tall fescue (stored for five years) and Russian wildrye. The positive effect of ultrasound was not significant on the germination of aged seeds of tall fescue (stored for one year) and perennial ryegrass, but was significant on the root length. Ultrasound treatment reinvigorated aged seeds by increasing superoxide dismutase (SOD) and peroxidase (POD) activities and reducing malondialdehyde (MDA) content. A regression model analysis was performed to determine the final optimal ultrasound condition for aged grass seeds that resulted in ultrasound time of 22 min, at 26 ℃, output power of 254.29 W and a seed soaking time of 2.89 h.