Abstract: Calcium treatment experiments were carried out on six heats of X70 pipeline steel.The results show that the Ca content of the steel dropped a lot and the composition and morphology of inclusions varied obviously in producing the steel after calcium treatment.Reoxidation deteriorates the effect of calcium treatment,but dissolved calcium and CaO-Al₂O₃ compound inclusions with high CaO content can play a role of modification on Al₂O₃ caused by reoxidation.The calcium treatment effect of high grade pipeline steel is related with the concentrations of calcium,oxygen and sulphur,reoxidation degree,and time after treatment,but evaluating calcium treatment with calcium content,[%Ca]_Tot/[%Al]_s,[%Ca]_Tot/T[O],ACR and [%Ca]_Tot/[%S] is improper.A evaluation standard is put forward as the following:(1) pure MnS inclusions do not exist in the central area of slabs or rolled plates,(2) n_CaO/Al2O3 of inclusions in the tundish and mould is controlled around that of 12CaO·7Al₂O₃,and (3) n_CaO/Al2O3 of inclusions after calcium treatment is higher than that of 12CaO·7Al₂O₃.It is also be careful that calcium treatment is carried out at the end of the final refining procedure so as to prevent reoxidation of liquid steel.