Physiological responses (cortisol, glucose, GPT, GOT, hematocrit, sodium, chloride, potassium, total protein and osmolality), growth and survival rates of grey mullet and Nile tilapia were tested by the gradual salinity changes for 70 days. Three different sizes of grey mullet, small (MS, $13.3pm1.8cm$), middle(MM, $28.9pm3.6cm$) and large (ML, $36.0pm2.0cm$), and three different sizes of tilapia, small (TS,$16.5pm1.7cm$), middle (TM, $20.6pm1.8cm$) and large (TL, $27.2pm2.7cm$)were used. Salinity of reareing water was increased $5textperthousand$ in every 5 days until it reached at $0 extperthousand$. The cortisol concentrations in all size groups of grey mullet were increased at both full strength seawater (SW) and fresh water (FW) at the end. The cortisol concentrations of TM and TL groups were higher in SW than FW (0 and 70 days). The GOT values of grey mullet were lower than those of Nile tilapia. The sodium concentrations of grey mullet in SW were ranged 160~184 mEq/$ell$, while those of Nile tilapia were the highest in the SW. No mortality was observed in MM and ML groups but survival rate of MS group was 76$ extperthousand$. Overall survival rate of Nile tilapia was lower than grey mullet. Survival rate of TS, TM and TL groups were $79 extperthousand$, $29 extperthousand$ and $55 extperthousand$, respectively.