Abstract: A new resonance scattering spectroscopic method for the determination of 2～40 ng/mL Si has been described. It is based on SiO₃^2-, Sb(Ⅲ) and MoO₄^2- forming the ternary heteropoly acid of Si Sb Mo in the medium of 0.04 mol/LH₂SO₄. After the concentration ofH₂SO₄ was adjusted to 0.22 mol/L, Rhodamine B was added. The associated complex of Si Sb Mo heteropoly acid rhodamine B was formed. It has a sensitive resonance scattering peak at 600 nm. The method has been applied to determination of Si in steel samples, with satisfactory results. The resonance scattering mechanism of the ternary associated complex has been investigated. The results show that the main reasion exhibiting the resonance scattering light is owing to the formation of unsoluble associated complex and colloid in big size.