Partially blocking the functionalities of polysiloxanediol and crosslinking agent by adding hexamethyldisiloxane in the silicone formulation, the cross-linking density of the active silicone layer in the silicone/PVDF composite membrane was reduced. The free volume and affinity of the active layer could further be modified by employing various tri-functional silanes as crosslinkers. The modified composite membranes were applied to a pervaporation process to separate ethanol-water mixture. The experimental results showed that the permeation rate was enhanced by hexamethyldisiloxane addition with small sacrifice of the ethanol permselectivity, especially in the case of low ethanol feed concentration. The membrane cured by a tri-fanctional crosslinker with attached phenyl group had better performance than tetra-fanctional one, in both selectivity and permeation rate.