Abstract: The thermodynamics and kinetics theoretical basis of modification of inclusions by calcium treatment is expounded. Through the test by LF-VD flowsheet,at calcium-free treatment conditions,except low carbon steel,the castability of medium and high carbon steel is good, the Ca content in the steel can be controlled by ≤0.001% ,and the hit rate of non-metallic inclusions of rating D and Ds W1,0 is obviously improved. By optimizing ingredient of refining slag and using a method of later sulfur-alloying for sulfur-containing aluminum killed steel, it can improve the morphology of inclusions, resulting in fine,diffuse distribution of ( Ca / Mn) • S inclusions to improve the cutting performance of steel.