Research on the effect of various chemical groups on the pervaporation performance of membranes was very important for understanding the separation phenomenon. In this article, a polystyrene membrane with -CF3 groups on its surface was prepared by casting normal polystyrene (PS-H) solution with small-molecular-weight alpha,omega-fluoroalkyl-terminated polystyrene (S-PS-(CF3)(2)) on a glass plate. The amount of -CF3 groups on the surface was adjusted by changing S-PS-(CF3)(2) content, which was documented by X-ray photoelectron spectroscopy analysis, surface energy, and contact angle measurements. The effect of S-PS-(CF3)(2) content on separation performance of S-PS-(CF3)(2) PS-H blend membranes for ethanol/water mixture was investigated. The results showed that water concentration in the permeate decreased with increasing S-PS-(CF3)(2) content; however, the permeation rate did not change until the S-PS-(CF3)(2) content was increased to 1 wt%. The reason for this was attributed to the fact that a small amount of SPS-(CF3)(2) enables a simple surface modification of the membrane keeping its inner structure relatively unchanged. This was confirmed by swelling degree measurements and other experiments. Comparison with our previous work showed that the -CF3 groups on the surface lowers the water permselectivity of the polystyrene membranes, whereas the -COOH groups enhance the water permselectivity. However, neither hydrophobic -CF3 nor hydrophilic groups on the membrane surface appeared to have any influence on flux.