A Pt—Ru nano-alloy/graphene catalyst comprises graphene as a support, and a Pt—Ru nano-alloy loaded on the graphene. The use of graphene as support for the catalyst takes advantage of the ion effect and tow-dimensional ductility of graphene, which increase the stability of the catalyst. The catalyst is prepared by a reverse micelles system method which provides a micro-environment (i.e. water-in-oil microemulsion), so that the particle size of the resulting nano-alloy particles can be regulated easily and is more uniformly distributed. The use of the catalyst in electrochemistry is also disclosed.