Abstract: Metalloscope, SEM and EDS (energy density spectrum) were used for the abnormal segregation band of X100 pipeline steel to observe its structure and analyze the composition linearly scanned fracture due to tension/impact and inclusions. The results indicated that the microstructure of the segregation band is polygonal ferrite with mixed crystal of which the sizes of some grains are different from each other, and the microstructure is greatly different from the matrix microstructure which is mainly the lath bainite. In the segregation band the carbon content is relatively low, while the manganese, silicon and oxygen contents are all relatively high. The layered fracture due to tension/impact is found around the center or at the quarter of the sectional thickness of the fracture, and an inclusion band is observed at the quarter of the whole thickness of impact fracture, where part of some inclusions are flaky. The impact toughness of specimens is thus seriously deteriorates.