The effect of $alpha-Tocopherol;(alpha-Toc)$ level in diet on the biochemical property of sweet smelt, Plecoglossus altivelis, was investigated. The cultured sweet smelt fish were fed two different diets for 8 weeks; a control diet was added $0.01\%$ of $alpha-Toc$ (CO group) and an experimental diet was added $1.00\%$ of $alpha-Toc$ (HT group). Both diets were rich in docosahexaenoic acid (DHA, 22:6n-3) and eicosapentaenoic acid (EPA, 20:5n-3), accounting for $12.3-13.2\%$ and $12.1-12.4\%$, respectively. Growth rate (GR) and feed efficiency (FE) were almost no difference between both groups, but female fish of both groups were superior to male fish in GR and FE. Lipid contents of muscle and ovary were high in CO group compared with HT group, but that of testis showed a similar level in both groups. The proportion of polyunsaturated fatty acids (PUFA) in muscle showed almost no difference in sex and between both groups. In testes, the proportion of PUFA was 1.35 times for HT group to 1.54 times for CO group as much as in ovaries, in which was high approximately 1.5 times compared with muscle. Thiobarbituric acid-reactive substances (TBARS) and hydroxyl (OH) radical levels of plasma were higher in CO group than HT group and superoxide dismutase activity was also slightly high in the former. The intensity of watermelon-like or cucumber -like aroma was related positively with TBARS and OH radical levels in plasma. The level of triglyceride (TG) and total cholesterol (CHOL) in plasma of CO group was higher than those of HT group. Survival rate was high in CO group with higher level of TG and CHOL in plasma.