Abstract: The number,size,chemical composition and morphology of inclusions in the sour service pipeline steel samples taken from individual steelmaking process stages and billet were analyzed using SEM and bulk sample electrolysis. The results indicate the area fraction of micro inclusions in the liquid steel decreases by 34.7% in the LF process; the area fraction of micro inclusions in the liquid steel of tundish increases by 6.1% compared with VD end.The dominant inclusion type is Al₂O₃ at LF start; The composite inclusions containing mainly CaO、MgO、Al₂O₃ form due to the steel/slag reaction in the LF and VD process.The amount-of-substance ratio of CaO to Al₂O₃ in the inclusions is close to 12 ∶ 7 after calcium treatment,desulphurization reaction between inclusions and liquid steel occurs and forms composite inclusions containing CaS.There are much more micro and macro inclusions in the billet at casting start stage compared with stable casting stage; T[O] content in the billet is less than 15×10^-6,and the macro inclusion content in the billet is less than 0.2 mg/kg in the stable casting stage; the macro inclusions in the billet are mainly from filler sand of ladle,mold powder.