The changes in mitochondrial ultrastructure, antioxidant enzymatic activities, lipid peroxidation and respiratory function of ultra-dried (4% moisture content) and artificially aged (48 days at 45°C) oat (Avena sativa) seeds were measured. The seeds were also primed with polyethylene glycol (PEG) solutions (0 and -1.2 MPa) for 12 hours at 20°C. The results showed that seed vigour, mitochondrial antioxidant enzymes including superoxide dismutase (SOD), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR), mitochondrial ultrastructure and their respiratory function in the embryo cells were improved with PEG-priming whilst mitochondrial H2O2 and malondialdehyde (MDA) contents decreased significantly (P < 0.05). This indicated that the effect of PEG-priming on seed vigour was positively related to mitochondrial antioxidant enzymes, mitochondrial ultrastructure and their respiratory function, but was negatively related to mitochondrial H2O2 and MDA contents. Compared with hydro-priming, PEG-priming was found to have a better effect on seed vigour in aged, ultra-dried oat seeds and this was closely correlated with the enhancements of mitochondrial antioxidant and respiratory function accompanying the regeneration of normal mitochondrial ultrastructure. Thus, ultra-dry and stored oat seeds should be primed with PEG solution at the appropriate water potential to alleviate the damage of seed ageing and imbibition.