Abstract: The high temperature tensile test of vanadium microalloyed beam blank S355ML was carried out. The microstructure at flange crack and fracture morphology at brittle temperature zone of the beam blank were analyzed by laser confocalujm microscopy, thermal field emission environmental scanning electron microscopy and scanning electron microscopy. The high temperature properties of the beam blank such as area reduction, tensile strength and the relationship between the microstructure proportion at fracture and area reduction were studied. The results show that at the strain rate of 1×10^-3 s^-1, the third brittle region is 700-900 ℃, and the plasticity decreases first and then increases with the increase of α-ferrite at fracture. When the test temperature is 850 ℃, the volume fraction of α-ferrite at fracture is 1.97%, and the reduction of area is 24.66%; When the test temperature is below 850 ℃, the reduction of area increases with the increase of α-ferrite. When the test temperature is 700 ℃, the volume fraction of α-ferrite is 64.51%, and the reduction of area also increases to 59.42%. According to the fracture morphology and reduction of area of tensile specimen, the best straightening temperature range of beam blank is 900-1 200 ℃.