Abstract: The composition segregation of the continuous cast round billets and the microstructure and phase composition of the quenching and tempering treated steel pipes made from the corrosion resistant Cr-Mo steel are investigated. The results reveal that there is some obvious composition segregation occurred at the center of the billet,some lump sulfides segregated at the grain boundary and some carbon-nitrides distributed along the intergranular structure in the segregation area,with the hardness of the segregation area higher than that of the matrix. At the center of the billet,the contents of such chemical composition as Mo,Cr,Mn,C and V in the segregation area are obviously higher than those in other area. The segregation phase analysis of the quenching and tempering treated steel pipes using a X-Ray Diffractometer indicates that M₃C,Mo₂C,M₃C₂ and MC are mainly contained in the segregation phase only with little M₂₃C₆ in it. The analysis using the Transmission Electron Microscopy shows M₇C₃ may also contained in the segregation phase of the steel pipe matrix in addition to M₃C,the layered tablets phase is Cr_15.58Fe_7.42C₆ and the regular hexagon phase is Mo₂C in the pipe inner wall.