Different sizes (statistical mean radius of GO nm(-1) mu m) of graphene oxide (GO) were synthesized from various sizes of graphite by the modified Hummers method. The GO obtained from the smallest size graphite had the smallest size and maximum oxidation degree under the same reaction condition. Different sizes of GO were incorporated into the sulfonated polyimide (SPI) to investigate the size effect on the structure and performance of composite proton exchange membrane (PEM) for direct methanol fuel cells (DMFCs). As the size of GO increased, the properties of SPI-GO composite membranes, such as proton conductivity, methanol permeability and mechanical property, presented a regular variation. Noteworthy, the SPI-0.5%-GO(1) composite membrane with the smallest size GO showed the best result, such as high conductivity (1.2 S cm(-1) at 80 degrees C and RH 100%), low methanol permeability (1.07 x 10(-7) cm(2) S at 25 degrees C) and outstanding fuel cell performance compared to that of pure SPI and other SPI-GO composite membranes. These excellent properties can be attributed to the formation of the well-defined microstructure and well-connected proton transport channels due to the strong hydrogen bonding interaction between the smallest size GO and SPI. Furthermore, the direct methanol fuel cell with SPI-0.5%-GO(1) membrane possessed a 1.4 times higher power density than pure SPI at 25 degrees C. (C) 2014 Published by Elsevier B.V.