A new sensor based on decorated copper nanoparticles and self-assembled graphene was fabricated and exemplified with the determination of nitrate solutions. Traditionally, graphene is coated on the sensor by drop-casting, leading to poor adhesion between graphene and the sensor. The self-assembled graphene proposed in this paper not only have a firm connection with the substrate, but also provide a three-dimensional network structure for copper nanoparticles. Copper was found as an effective catalyst for nitrate reduction. The combination of copper nanoparticles and self-assembled graphene can greatly enhance the sensitivity. Thus, low detection limit of 7.89 µM is obtained for nitrate, which to our knowledge, is among the lowest reported in the literatures. This method was employed for the determination of nitrate in lake water and the results were in good agreement with those obtained from a standard analytical procedure.