Abstract: A study has been undertaken to explore the effects of polyethylene glycol (PEG) priming on the vigor of aged oat seeds (Avena sativa). The research also aimed to determine the optimal PEG concentration and priming time. Seeds with ultra-dry moisture content (4%) were deteriorated for 48 d at 45℃. They were then primed for 0 (control), 3, 6 and 12 h with different concentrations of PEG-6000 (0, -0.3, -0.6, -0.9 and -1.2 MPa). The germination percentage, germination index, mean time to germination and seedling vigor index were then analyzed. The results showed that the germination percentage, germination index and seedling vigor index of aged oat seeds decreased after priming with low concentrations of PEG (0--0.6 MPa) in relation to the control, but that mean time to germination increased. However, the opposite trend was observed after priming with high PEG concentrations (-0.9 and -1.2 MPa). These results indicate that PEG priming not only prevents the imbibition damage for oat seeds with ultra-dry moisture content but also repairs damage from seed ageing. The effect of PEG priming was closely related to its concentration, priming time and their interaction. This study determined that the appropriate concentration and priming time was -1.2 MPa and 12 h for aged oat seeds. However, further research is needed to confirm whether these levels can be practically applied in agricultural production.