BACKGROUND: Stocking density induced stress that affects energy intake, growth rate, enzymatic activity and oxygen consumption in fish. OBJECTIVES: In this study the development of antioxidant enzyme Glutathione peroxidase (GPX), catalase (CAT) and superoxide dismutase (SOD) in egg and larvae of rainbow trout (Oncorhynchus mykiss) that kept in different stocking density was investigated. METHODS: Four groups of eggs, cultured in pans (size16×9.5×10 cm, volume 10 L) with density of 400egg/l (group1 as routine density), 200egg/l (group2 as low density) and 600egg/l (group3 as high density) in triplicate. Eggs were held in tanks supplied with flow-through freshwater at 10.8oC. samples were taken at day1 (fertilization), 3 (Cleavage), 8(organogenesis), 16 (eyed egg), 31 (hatch) and 48 (active feeding). RESULTS: A significant increase in GPX and SOD activity was seen from fertilization till eggs were eyed. (p<0.001). The activity of these enzymes decreased to active feeding (p<0.001), meanwhile this activity was higher than the fertilization starts. Similar changes were seen in SOD and GPX activities in higher and lower densities (Group 2, 3). CAT activity increased from fertilization to organogenesis and then toward active feeding significantly decreased in compare to fertilization (p<0.001). Pattern of changes of catalase activity in high density group was similar to routine density but in low density group it was higher until eggs were eyed. GPX, SOD and CAT activities did not show significant difference in similar days among different groups (p<0.05). CONCLUSIONS: Our results showed that rearing densities used in this study could not affect antioxidant defense development in early life stages of O. mykiss.