Abstract: Internal folds of seamless steel pipes fall into various types due to rather complicated causes for such defect. CC round central quality vs the fold type is studied with acid etch macrostructure tests as conducted to CC billet,seamless steel pipe and the shell from some rolling failure. As a result,it is identified that billet center with rich dendrites is apt to produce central cracks which lead to pipe internal folds featuring cracks running inward along with the dendrites,while billet center with serious porosity is apt to develop spotted and hair-lined clearances which lead to pipe internal folds as laminal cracks parallel to the inner surface. Accordingly, the mould-electric magnetic stirring (M-EMS) process is used to enlarge the billet central equiaxed zone,having got the internal fold rate of the 20 pipe reduced from the original 0.22% to 0.07%.