Abstract: Influence of deformation temperature and reduction on the microstructure of SS400 low-carbon steel after hot compression deformation was studied by Gleeble 3500 thermal simulator. The results show that the microstructure of the SS400 steel was composed of ferrite and pearlite, the ferrite grains were mainly characterized as acicular, equiaxed and long banded corresponding to the deformation temperatures as 850, 800, 775 and 750 ℃ and the reduction as 20%, 40% and 60%. The ferrite grain size decreased and the volume fraction increased with the decrease of deformation temperature and the increase of reduction, the maximum was 79.2%, and the minimum was about 45%.