Velocity and temperature distribution of molten steel in mold for slab with nozzle submerged depth 150-450 mm have been simulated by using software CFX4.3 and a new method to give the velocity at inlet of mold. Results showed that with increasing nozzle submerged depth

the basic pattern of flow and temperature Held of steel didn't change

but the upper backflow region enlarged and the center of lower backflow region moved down； the value of down- and upward maximum shearing stress at narrow face increased and the position of occurred peak value moved down； the position of peak heat flux on narrow face moved down

and with submerged depth 300 mm

the peak value was minimum； the position of impingement point also moved down； and the horizontal velocity of molten steel in meniscus zone and the F number which expresses free surface fluctuation decreased. For 1 200 mm x 250 mm slab with casting speed 1.0 m/min and nozzle port angle 15°

the optimum nozzle submerged depth is 300 mm.