Inclusion morphology, composition and number at different times of RH vacuum treatment were investigated on API-X70 pipeline steel with ASPEX. It is found that the inclusions are mainly globular CaO–Al2O3–MgO. The inclusion number decreases with time during RH treatment and the removal ratio can reach 72% after 28 min treatment. The initial number of inclusion plays a vital role in final inclusion number, so efforts must be made to eliminate the inclusion before RH treatment to get a highly clean steel. Non-liquid inclusions including C3A (C is CaO and A is Al2O3) and Al2O3 · MgO decrease much more quickly and thoroughly than liquid ones, C12A7. This can be explained by the fact that non-liquid inclusions have a much higher contact angle and interfacial energy but a much lower work of adhesion than liquid ones between inclusion and steel melt.