Abstract: The bio-desulphurization experiments on slag were carried out. The results show that pulp density, Fe³⁺ concentration and pH are effect on Thiobacillus ferrooxidans(T.f.) consistency free in the solution and desulphurization rate. The bio-desulphurization from slag is due to the combination of the direct bacterial action on the mineral surface with the indirect leaching by the ferric ion produced by the bacteria into the solution. Desulphurization rate and oxidative activity of the T.f. are influenced by the growth of T.f adhered to the mineral, mineral density, Fe³⁺ concentration and pH consistency. The addition of ferric ion can interfere in the bacteria oxidative activity and result in inhibiting effect on the desulphurization, some sediment on the surface of the mineral are formed to reduce desulphurization rate.