This paper reports several physiological and biochemical changes related to active oxygen injure and scavenging effect in a low temperature resistant strain Isochrysis galbana Parke 8701 under low temperature stress and investigates its low temperature tolerant mechanism in comparison with a low temperature sensitive strain I. galbana Parke 3011. The experimental results showed that : ⑴the activity of superoxide dismutase (SOD) in I. galbana Parke 8701 decreased slightly when cultured at low temperature ( 8 ℃) for a week , but still kept extremely high enzymatic activity , so it could scavenge the extra active oxygen rapidly under low temperature stress and keep a membrane lipid peroxidation in relatively lower level as well as a relatively low electrolytic leakage; ⑵the membrane lipid peroxidation level and electrolytic leakage in I. galbana Parke 3011 were significantly higher than in I. galbana Parke 8701 when stressed under low temperature for a week. This was due to the relatively low SOD activity in the former and its activity rapidly declined; ⑶in comparison with SOD , peroxidase (POD) did not play an important role in relieving the low temperature injury and raising the capacity of low temperature tolerance in these two strains of I. galbana Parke.