Objective：To observe the effects of mitochondrion-targeted antioxidant SS-31 on hippocampal mitochondrial morphology and cognition after isoflurane-anesthesia in developing rats. Methods：Seventy-two postnatal day（PND）7 rats were equally divided into 3 groups（n=24）：oxygen + normal saline（the CN group），isoflurane + normal saline（the IN group），and isoflurane + SS-31（the IS group）. SS-31（5 mg/kg）or normal saline was intraperitoneally administered with a volume of 0.4 mL/kg 30 min before gas inhalation. Malondialdehyde（MDA）content，reactive oxygen species（ROS）levels，and superoxide dismutase（SOD）activities were determined on PND7. Mitochondrial morphology was determined on PND21，and behavioral studies were performed on PND40 or PND60. Results：Compared with the IN group，the levels of ROS and MDA，and number of abnormal mitochondria were decreased，while the activity of SOD，times of crossing-platform and time spending in target quadrant were increased in the IS group. Conclusion：Mitochondrial antioxidant SS-31 attenuates isoflurane-induced oxidative stress，and impairments of mitochondrial morphology and cognition in developing rats.