Field experiments were conducted in 3 rice seasons (from summer 2009 to summer 2010) to study the effect of steel-making slag to methane (CH4 emission in paddies of Dystric Fluvisols (Hai Duong) and Eutric Fluvisols (Hanoi). The results showed that soil applied with steel-making slag at rate of 1,000 kg/ha gave seasonal CH. emission equalled to 62-74 percent of that of control treatments. Rice yield of treatment applied with steel-making slag in Hai Duong was 5-15 percent higher than that of control and varied from 5.3 to 6.4 tons/ha. In Hanoi at spring season 2010, treatment applied with steel-making slag had rice yield 14.7 percent higher than that of control, valued from 4.3 to 5.5 tons/ha. The results showed that steel-making slag had more effect to rice yield and CH. emission in Hai Duong soil more than Hanoi soil. Treatment with drainage had possitive effect to mitigation of CH. but not so clearly as treatment with steel-making slag. CH. suppression mainly due to Fe content in steelmaking slag as Fe become oxidation substance in submerged rice soil. In the other hand, CH. emission was also reduced when soil is drained resulted in higher oxidation level in soil.