Abstract: The aims of this study were to evaluate the effects of stocking density of Oncorhynchus mykiss that were reared in net cages on free triiodothyronine (FT3), free thyroxine (FT4), triglyceride (TG) and total cholesterin (TC) levels, and the expression of IGF-I and TR-. The results showed that FT3 and FT4 levels decreased gradually as the stocking density increased, and they were positively associated with the culture time before November. TG levels decreased as the stocking density increased while TC levels increased as the stocking density increased and both TG and TC levels were negatively associated with the culture temperature. Moreover, TG levels in the treatment group 3(GR3) were significantly lower than those in the treatment group 1(GR1) in November, and significant differences were observed among the three treatments in January of the following year (P 0.05). Additionally, the expression levels of IGF-I and TR- decreased gradually as the stocking density increased. These results indicate that high stocking density could be a stressor for O. mykiss, that nd that may reduce FT3 and FT4 levels in the blood and then affect the growth and development of fish, which is supported by the trend of special growth rate (SGR). There was a relationship between thyroid hormone and the expression of IGF-I, The levels of thyroid hormone was related to the expression of IGF-I; however further investigation is needed to study whether thyroid hormone regulates the expression.