Using a repeating deoxidization method, we prepared Au nanoparticles with different mean sizes of 11 nm, 35 nm and 58 nm. Then the Au nanoparticles were dispersed in mineral oil to form Au nanofluids by the surfactant-assisted phase transition technique. Under electric field, the optical properties of the Au nanofluids were measured. It was found that the Au nanofluids exhibit significant electric field-induced birefringence and the birefringence index can be adjusted by changing the external electric field strength. Furthermore, the birefringence index is influenced by the Au concentration and particle size. Within the suitable particle concentration range, the index decreases with the increase of Au concentration and particle size. Finally, we further discuss the electric field-induced birefringence phenomena of the Au nanofluids according to the structure transformation mechanism of electrorheological fluids under electric field.