Abstract: The purpose of this study was to investigate dietary lipid requirement and the effect of different lipid levels on the body composition and plasma biochemical indices in cobia (Rachycentron canadum L.) at two different body sizes (initial weight: 40 g vs. 500 g). Six isonitrogenous diets were formulated using fish meal, soybean meal and wheat meal as the protein source and fish oil as lipid source, with six different lipid contents of 5.27%, 8.22%, 10.81%, 14.26%, 17.32% and 20.94% (at a dry matter basis), respectively. Fish at two different growth stages were randomly distributed to seawater floating net cages (1.5 m×1.5 m×2.5 m) at a density of 25 and 10 fish per net cage, respectively. Each diet was fed to triplicate groups of fish twice daily to satiation for 10 weeks (40 g) and 8 weeks (500 g), respectively. Specific growth rate (SGR) and feed efficiency ratio (FER) significantly increased and then decreased with increasing dietary lipid levels at both developmental stages. The Viscerosomatic index (VSI) and Condition factor (CF) increased significantly with the increase of dietary lipid levels. Significantly higher Hepatosomatic index (HSI), whole-body, liver and muscle lipid contents and lower moisture and protein contents were observed in fish feed the diets containing higher lipid among small fish groups, but no significant difference was detected among large body size groups among above parameters. In addition, the contents of TC, TG, HDL-C, LDL-C and Glu increased significantly with the increasing lipid levels among small fish groups, there was no significant effect among large fish groups in these indices except TG. The overall results showed that based on SGR and FER, the diet containing 13.97%—14.16% and 13.18%—13.47% lipid was most suitable for the cobia culture at the body size of 40 g and 500 g, respectively.