Abstract: High temperature mechanical properties of Q345B and Q345C steel continuously cast slabs were tested by Gleeble-1500 thermal simulation machine.The morphology of the fracture surface was analyzed by SEM.And the embrittlement mechanism was studied.The results show that the embrittlement temperature region Ⅲ of the Q345B steel was 700-825 ℃,and the Q345C steel was 600-980 ℃.Tensile strength of both the steels reduced with increasing the temperature in 600-1 350 ℃,and the reduction of area of Q345C steel was less than 30% in 780-840 ℃ and Q345B steel was always higher than 30% in all temperatures.Both the two steels occured transfusion fracture at 1 350 ℃,and plastic transcrystalline fracture at 1 000 ℃.The Q345B steel still occured transcrystalline fracture at 900 ℃,while the Q345C steel occured mixed fracture of transcrystalline and intergranular fracture at 900 ℃.The fracture mode of the two steels changed from brittle fracture at 800 ℃ to plastic fracture at 600 ℃.Causes of embrittlement of the Q345 steel existed in two aspects,one was that superfine Nb(CN) second phase particles precipitated at the austenite single phase region led to the embrittlement occured which was resulted from the strain concentration at the grain boundary,and the other was that the strength of reticulate ferrite precipated from original austenite grain boundaries was low,that resulted in the embrittlement.