The aim of this study was to evaluate the effects of Artemia nauplii enriched with different concentrations of highly unsaturated fatty acids (HUFAs) on the lipid metabolic response, peroxidation, and antioxidant defence status of the lined seahorse (Hippocampus erectus) juveniles. Twenty-day-old juveniles were fed Artemia nauplii enriched with four different concentrations (0.0 µL/L [control, A], 13.5 µL/L [B], 27.0 µL/L [C], and 54.0 µL/L [D]) of HUFAs (two thirds DHA and one third EPA) for 30 d. The activities of lipase and lipoproteinlipase of the juveniles significantly increased with increasing HUFA concentration; however, the activities of malate dehydrogenase and lactate content decreased with increasing HUFA concentration. Alkaline phosphatase activity and pyruvic acid content were not significantly different among the three treatments and the control. Malonaldehyde content was significantly negatively related to the enrichment concentrations (in Treatments A, B, and C); however, it increased significantly in Treatment D. The activities of superoxide dismutase in Treatments A and B were significantly higher than that in Treatments C and D. Catalase activity increased significantly from the control to Treatment C, then decreased significantly in Treatment D. Glutathione peroxidase activities increased significantly with increasing concentration of HUFAs, and peaked in Treatment D. The results indicate that dietary HUFAs are able to modify some enzymatic activities, and moderate dietary HUFA supplementation significantly promotes lipid metabolism and reduces lipid peroxidation products by enhancing antioxidant defence in the juveniles. However, excess HUFAs may result in adverse effects on the enzymatic activities in the juveniles, which might be related to oxidative stress.