Pt–graphene (Pt–G) is prepared by in-situ reduction of Pt precursor on graphene oxide surface using a microwave method. Various Nafion/xPt–G/ySiO 2 composite membranes where x is 0.5wt%, 1.5wt% and 3.0wt% and y is 0.0wt%, 1.5wt% and 3.0wt%, respectively are fabricated of which water uptake and proton conductivity show similar trend: that is, those are increased with both Pt–G and SiO 2 contents. The cell performance is significantly affected by relative amount of Pt–G and SiO 2 in composite membrane. Below 1.5wt% in Pt–G content, the cell performance is increased with SiO 2 due to the excellent retention ability of SiO 2 for water produced from Pt site on graphene. Beyond 1.5wt% in Pt–G content, however, the cell performance is rather decreased compared to that of 1.5wt% Pt–G. It is ascribed to the blocking effect for proton conduction due to too much inorganic filler and possible loss of electrons through Pt network. The effect of SiO 2 on cell performance seems to be different depending on Pt–G and relative humidity (RH). Open circuit voltage (OCV) is significantly affected by SiO 2 content and increased with SiO 2 content.