Abstract: High-strength IF steel with P addition was studied.The strain-stress curves were measured using Gleeble 3800 thermal mechanical simulator at strain rates of 0.01,0.1,1,10 s-1 with 50% reduction and at deformation temperatures of 950 ℃ and 850 ℃.The single pass compression test shows that the mode of stress-strain curves is dynamic recovery with strain rate 10 s-1 and 50% reduction,and it do not change with temperature.The grain is large with the strain rate increasing at deformation temperature of 950 ℃,but difference is small at deformation temperature of 850 ℃.The deformation temperature is the main factor which affects the grain size of austenite or ferrite when the strain rate is not changed largely.