Abstract: The effect of gas flow rate (0 - 10 L/min) and nozzle bottom shape ( concave bottom and pyramid bottom) on flow field in mold has been studied by using 1: 1 water model. In condition of mold section 230 mm x 1 200 mm and casting speed 1. 6 m/min the fluid pattern in mold with using concave bottom nozzle is better than that with using pyramid bottom nozzle ； and the stability of liquid level in mold with using concave bottom nozzle is also better than that with using pyramid bottom nozzle; with gas flow rate 0 〜8 L/min, the liquid surface velocity with using pyramid bottom nozzle is obviously higher than that with using concave bottom nozzle ； for using concave bottom nozzle as gas flow rate is larger than 8 L/min, its surface velocity is lower than 0. 2 m/s； 120 heats IF steel commercial production results show that using optimized concave bottom nozzle with argon flow rate 7 L/min the fluctuation of surface of liquid in mold during casting precess is within ± 3 mm, and compared with before optimization the inclusions amount in casting slab decreases by 24%.