Steel and slag samples were taken at the start and the end of LF refining for steel plate cold common(SPCC),in the compact strip production(CSP)process,and at the same time,the temperature and oxygen activity a[O]were measured by using an oxygen sensor.Furthermore,inclusions in steel samples were monitored by scanning electron microscopy(SEM)combined with energy dispersive spectroscopy(EDS).It was confirmed that a[O]in liquid steel was in equilibrium with inclusion rather than with top slag during LF refining.Desulfurization was related to deoxidation since a[O]at slag-steel interface was clarified to be very close to that in liquid steel under the specific condition in LF with intense stirring by argon blowing and refined by highly basic low oxidizing slag for Al-killed steel.Sulfur partition ratio(L S)was very sensitive to a[O].Since a[O]increased rapidly with temperature rise,it not only offset promotion to desulfurization reaction with temperature rise but decreased LS.For Al-killed steel,the modification of Al2O3for lowering the activity of Al2O3in inclusion was believed to be favorable for both deoxidation and desulfurization during LF refining. Steel and slag samples were taken at the start and the end of LF refining for steel plate cold common (SPCC), in the compact strip production (CSP) process, and at the same time, the temperature and oxygen activity a [O] were measured by using an oxygen sensor. Future inclusions in steel samples were monitored by scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (EDS) .It was confirmed that a [O] in liquid steel was in equilibrium with inclusion rather than with top slag during LF refining. Desulfurization was related to deoxidation since a [O] at slag-steel interface was clarified to be very close to that in liquid steel under the specific condition in LF with intense stirring by argon blowing and refined by highly basic low oxidizing slag for Al-killed steel. Sulfur partition ratio (LS) was very sensitive to a [O] .ince a [O] increased rapidly with temperature rise, it not only offset promotion to desulfurization reaction with temperature rise but decreased LS.For Al-kille d steel, the modification of Al2O3 for lowering the activity of Al2O3 inclusions was believed to be favorable for both deoxidation and desulfurization during LF refining.