Abstract: Described in the assay is the morphology of the surface flaws of the Gr6 billet for making of the cryogenic service seamless steel pipe. Moreover,the causes for formation of the said flaws are investigated,and then relevant preventative actions are put forward. The investigation leads to the following conclusion. During the solidification process,δ-Fe →γ-Fe transformation occurs with the initially-solidified shell of the mould meniscus region along with dropping of the temperature. Meanwhile,with considerable volume and linear shrinkages,the shell breaks away from the mould copper surface so early that air gas is formed,and this leads to uneven growth of the shell,and flaws in turn develop in the weakest area of the shell. Accordingly,occurrence of flaws of the billet for making the cryogenic service seamless steel pipe could be properly controlled by means of such actions as enhancing steel cleanness,well controlling overheat of the hot metal, reducing withdrawing velocity, appropriately controlling fluctuation of the mould level,and selecting suitable mould tamper,etc.