Epoxy based nanocomposites using graphene oxide (GO) sheets dispersed multi-walled carbon nanotubes (CNTs) as combination fillers were prepared using an in situ polymerization technique. A remarkable synergetic effect was observed between CNTs and GO sheets which improved the mechanical properties of the epoxy. It was confirmed by optical and field-emission scanning electron microscopy (FESEM) images that the dispersion of CNTs in epoxy matrix can be significantly improved by adding GO sheets. The overall mechanical properties of CNT–GO/epoxy composites were greatly enhanced with only adding 0.04 wt% (percent by weight) CNTs and 0.2 wt% GO sheets. Moreover, the fatigue and creep rupture lives of pure epoxy was also significantly increased by the addition of GO dispersed CNTs. Approximately a 950% improvement in fatigue life, and 400% improvement in creep rupture life were observed at the applied stress levels tested.