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Thyroid hormones (T3, T4) have marked effect on body metabolism and in controlling osmoregulation activity in fish. The aim of this study was to determine the effect of water salinity changes on thyroid hormones level and thyroid-stimulating hormone (TSH) of grass carp. For this purpose 120 grass carp were divided randomly in to four groups (10 fish in each group and three replicates per treatment). Three groups were held in three different salinities at concentrations of 4, 8 and 12 g L-1. The fourth group was reared in fresh water and considered as control. After three weeks blood samples were collected from the caudal peduncle vein. Then serum was separated and serum thyroid hormones and TSH were measured by LISA on Microwell plates. Our results indicated that the average of T3 levels in 4, 8 and 12 g L-1 groups were 0.43 ± 0.11, 0.22 ± 0.04 and 0.21 ± 0.04 μg dL-1, respectively. T3 levels in all experimental groups were significantly lower than those of control group (p < 0.05). Serum T4 level in 4, 8 and 12 g L-1 groups were 0.29 ± 0.06, 0.24 ± 0.05 and 2.85 ± 0.55 μg dL-1, respectively. Thyroxine level was significantly higher only in 12 g L-1 group in comparison with the control and other experimental groups (p < 0.05). Thyroxine level in other groups had not any significant difference with the control group (p> 0.05). The level of TSH in salinities of 4 and 8 g L-1 groups was significantly higher than that of control group (p < 0.05). The results showed that increasing water salinity can have significant effect on thyroid activity by decreasing T3 and increasing T4 level in serum of grass carp in experimental condition