A polydimethylsiloxane (PDMS) membrane was improved by graft polymerization of 1H,1H,9H-hexadecafluorononyl methacrylate (HDFNMA) by plasma, which had the effect of increasing the selectivity for volatile organic compounds (VOCs). The use of an easy quantitative analysis for the pervaporation through plasma-grafted PDMS membranes was investigated. The degrees of grafting on the inside and reverse side of the grafted PDMS membranes were lower than on the surface. Only part of the HDFNMA sorbed into the PDMS membrane was grafted onto the PDMS membrane. The relationship between the feed concentration and the permeate concentration was observed to be linear. The pervaporation through the grafted PDMS membrane could be used for easy quantitative analysis. The solubility of VOCs for the grafted PDMS membrane was high when compared with the solubility for the PDMS membrane. The grafted PDMS membrane that had high VOC concentrations of the sorbed solution showed an excellent separation performance.