Abstract: Chemical composition,fracture features and microstructure of TB5 slab were investigated.The results indicated that chemical compositions of slab are uniform and meet the national requirements.The fracture feature of slab was cleavage or quasi-cleavage.There were precipitated phases in the fracture,but no inclusions in it.The microstructure of slab was rough equiaxed grains,in which there are long needle-like precipitation phases,and the distribution of the phases are uniform.The analysis results showed that the slab fracture was not related to chemical composition but to the rough grains and the coarse precipitation phases existed in the grains,which cut the β-matrix apart and impeded dislocation movement,therefore reduced greatly plasticity of TB5 slab.