The desalination performances of graphene oxide (GO) membranes were further optimized using selective removal of epoxy groups and increase of carboxyl groups on the GO surface. For this purpose, the carboxyl functionalized GO nanosheets, which was named as graphene acid (GA), was prepared according to multiple oxidation technique via Hummer's method. The graphene acid (GA) membrane was then successfully fabricated using pressure assisted self-assembly (PASA) method and its properties were compared with pristine GO membrane, amine funcdonalized GO membrane and other carboxylation GO membrane via nucleophilic substitution reactions method, such as amino poly-carboxylic acid (APCA) and amino acid (AC) functionalized GO membranes. The results showed that GA membranes exhibited not only higher permeability but also better salt rejections compared to pristine GO and other functionalized GO membranes because of the improved physicochemical properties of GA membrane, including surface negative charge and hydrophilicity. The GA membrane exhibited a pure water flux of 73.2L.m(-2).h(-1) and maximum rejections of 64.4% and 95.6% for 2000 mg/ L of NaCI and Na2SO4 aqueous feed solutions, respectively at 1.5 MPa. These results were higher than those of the pristine GO and functionalized GO membranes. Characterization of GO and GM were also done to provide more scientific explanation on the experimental results.