Abstract: The morphology, distribution, and entrapped inclusions of bubbles in thick slab of X70 pipeline steels were investigated by peeling layer.The results show that the diameters of 94%bubbles in thick slab are less than 200μm;the bubbles and the entrapped inclusions are common in 8 mm depth from the slab surface, but the defects are hardly found as peeling deeper into the middle of slab, only a small number of defects are shown, presenting a random distribution in this area;the main morphology of bubbles existing in slab are single bubbles and twin bubbles, the calabash bubbles and petal bubbles are also found;the inclusions are mainly in four existent forms:oxides, aggregation in clusters, bubbles in clusters, and ash inclusions.