Abstract: Gold-silver alloy nanoparticles were synthesized in aqueous solution by the co-reduction of chlorauric acid HAuCl₄ and silver nitrate AgNO₃ using sodium citrate as reducing agent. TEM was employed to analyse the morphology and size of the alloy nanoparticles. The optical-absorption spectra in the range of 300-800nm show the characteristic of single plasmon resonance absorption band,and the absarption band shows blue-shift with decreasing the molar ratio of HAuCl₄ to AgNO₃ in the reacting solution. The experimental results show that the gold-silver alloy nanoparticles possess composition-dependent tunability of their optical property which makes them have potential applications in the field of nano-optics.