Graphene oxide (GO) prepared in bulk quantities by oxidation of graphite with strong oxidants contains many hydrophilic groups, such as hydroxyl, epoxy, and carboxyl acid. We present a method to efficiently convert these hydrophilic groups into alkyl and alkyl ether groups by a one step reaction of bimolecular nucleophilic substitution with alkyl bromide. The functionalized graphene oxide (fGO) can be homogeneously dispersed as exfoliated monolayers in various organic solvents without degradation of size and shape of graphene oxide sheet. The degree of substitution reaction of each hydrophilic group in GO with alkyl bromide is quantitatively determined by comparing the deconvoluted O 1s X-ray photoelectron spectrum of GO with that of fGO. Addition of a small amount of fGO in poly(ethylene terephthalate) (PET) improves remarkably tensile and gas barrier properties of PET/fGO composite due to homogeneous dispersion of fGO sheets in PET matrix.