A series of Ag films with different thicknesses were prepared on polyethylene terephthalate (PET) substrates under identical conditions by thermal evaporation. The effect of the thickness on the optical and electrical properties of the films was studied. The morphology of the samples was investigated by atomic force microscopy (AFM). The optical and electrical properties were measured by spectrophotometer and four-point probe method, respectively. The experimental results show that the reflectance increases, while transmittance and resistivity decrease with the increase of the thickness. There exists a critical thickness of the film and it is 75 nm in this experiment. The optical and electrical properties of Ag films on PET substrates with thickness larger than critical thickness, are close to those of the conventional bulk silver. The resistivity of the 150-nm film is 3.0±0.2 μΩ•cm, which is lower than that of the 250-nm Ag film grown on BK-7 glass substrates.