In order to investigate behaviour if simultaneous dephosphorization and desulfurization three kinds of 5㎏ lots of molten pig iron samples were prepared under Ar atmosphere as follows : a) primarily, pig iron of α_0=f_0[%O] =1.5×10^(-4), and [Si]=0.84%, b) second samples of [Si]=0.05% and α_0= 2.1 ×10^(-4) by addition of ferric oxide to the first sample as much as 12g/㎏-H.M. and holding for 30 min, c) sample of [Si] =0.03% and α_0=2.4×10^(-4) by charging a double quantity of ferric oxide to the first sample. As much as 10 and 20g of soda ash was charged from the top of the respective melt and the change in the composition of the each melt was traced as a function of time. The experimental results are summarized as follows. 1. The rate of dephosphorization at slag/metal interface is proportional to the difference between [P] and [Si] : (수식) 2. The oxygen activity of the melt decreased rapidly and exhibited minimum values, the level of which lowered as the silicon content of the melt was greater. 3. In case if [Si]=0.084%, the desulfurization presumably proceeded according to the reaction, (Na₂O)+[C]+[S]=(Na₂S)+CO ; and [S] reached to a level of 0.001% after holding 30min. on addition of soda ash.