Epoxy/polyethersulphone (PES) blends are prepared as matrix resins for composites using carbon-based nanofillers as functional fillers. The epoxy/PES blends form a network structure via reaction-induced phase separation (RIPS) during the curing process, and the nanofillers are selectively localized in the phase inverted PES phase and at the interface leading to a three-dimensional continuous filler network. With this unique structure, the thermal conductivity, electrical conductivity and mechanical properties of the epoxy/PES composite are improved, and they are correlated to the final morphology. The structural development of the composites is also discussed. This work may provide a simple and efficient method without solvent to construct high-performance, functional epoxy based composites.