Silver nanofluid is prepared by a micro-emulsion method.The birefringence of this silver nanofluid is investigated by a He-Ne laser (λ = 632.8 nm).The wave vector of optical wave is perpendicular to the direction of applied electric fields,and the angle of incidence thought fluids is about 5°.The results show that the silver nanofluid without electric fields does not exhibit the birefringence,while the birefringence appears when the electric field is applied to the silver nanofluid.For the same electric field,the refraction indexes of both o-light and e-light increase with the increment of silver concentration.But,the refraction indexes of nanofluids with a fixed silver concentration decrease with the increment of electric field strength.The results show that the change of refraction index of e-light is more than that of o-light.