Most of magnesium (Mg) in fish is contained in the bone. Dietary magnesium has been reported to affect growth in several species, but the effect of dietary Mg on growth of seabass (Lateolabrax japonicus) is unknown. This study was conducted to investigate the effect of dietary Mg supplement on growth, feed efficiency, morphometry index, and Mg concentration in vertebrae, muscle and serum of seabass. Six experimental diets were formulated to contain graded levels of Mg by supplementing the basal diet with 186, 350, 542, 735, 950, and 1220 mg/kg Mg in the form of Mg sulphate (MgSO4·7H2O). Triplicate groups of juvenile seabass with an initial body weight of (28.0±2.5) g were fed to apparent satiation twice daily for 56 d with the waterborne Mg concentration of 1115 mg/L. The experiment was carried out in flowing-water tanks. Dietary Mg supplement did not improve the specific growth rate, survival rate and feed efficiency of seabass (P>0.05). The hepatosomatic index, viscerosomatic index, condition factor, Mg concentration in vertebrae, muscle and serum were not affected by dietary Mg supplementation(P>0.05). Also crude protein, crude lipid, moisture and ash contents in muscle were no significantly different (P>0.05). These indicates the Mg requirement of sea bass was met by fish fed the basal diet and waterborne Mg concentration. Therefore, it is no need to additional magnesium supplementation in feed