Abstract: The high temperature ductility of Nb microalloyed 55CrV steel was determined by using Gleeble-3500 thermal simulation testing machine. The morphology of the fracture and the microstructure near the fracture was observed by OM and SEM. The precipitation of the second phase was simulated by Thermo-calc software to analyze the high temperature plasticity mechanism. The results show that there are two brittle intervals for the Nb alloyed 55CrV steel and the temperature of first brittle zone is 1300-1245 ℃ and the third brittle zone is 849-600 ℃. The precipitation of fine niobium-vanadium composite precipitate retards the occurrence of dynamic recrystallization at high temperatures. At 850 ℃ below, the precipitation of vanadium carbide, niobium carbide and cementite reduces the thermoplasticity of the steel. In order to avoid the occurrence of cracks in the continuous casting billet, the temperature should be controlled above 850 ℃ at straightening point.