Abstract: The microstructure evolution, precipitation behavior of carbonitride as microalloying element and its effects on deformation induced ferrite transformation (DIFT) of mild steel with various Nb, V (N) content were investigated on a Gleeble 1500 thermomechanical simulator through multi-pass thermocompression in the temperature range of 820-790°C to simulate the finish rolling process of plates. The results showed that part of austenite is transformed into ferrite through DIFT with reversed or meta-dynamic transformation found at intervals of deformation and depending on its temperature; in the microstructure the mean grain size is less than 5μm through the quick chilling after multi-pass deformation; DIFT is inhibited by adding V only to the steel to get the microstructure refined, while adding V-N compound to the steel will enhance DIFT to coarsen the microstructure; adding Nb to the steel will not inhibit DIFT but get the microstructure refined remarkably; the slower the chilling rate after deformation, the more apparent the difference between such steel. It was revealed that the effects of microalloying elements are correlated to the dissolution and precipitation behavior of carbonitride.