Abstract: In sulfuric acid-thiourea-potassium thiocyanate medium, nanogold in size of 70 nm exhibit weak resonance scattering signal. Mo(Ⅵ) is reduced to Mo(Ⅴ) by thiourea, and Mo(Ⅴ) reacts with potassium thiocyanate to form a orange-red complex . When the complex coexists with gold nanoparticles, there are two resonance scattering peaks at 402 nm and 554 nm respectively. Mo(Ⅴ) concentration in the range of 1.0~20×10^-6mol/L is proportional to the enhanced resonance scattering intensity at 402 nm, with detection limit of 5.10×10^-7 mol/L Mo. This method was applied to determination of Mo in waste water, with satisfactory results.