The effects of graphene nanosheets (GNS) and layered double hydroxides (LDH) on morphology, flame retardancy and thermal degradation of epoxy resin (ER) were investigated. LDH was exfoliated in ER/GNS/LDH nanocomposites, while GNS was partially exfoliated and partially agglomerated into small clusters. Since not all of the GNS had good dispersion, the compactness and barrier properties of residual char in ER/GNS/LDH were inferior to ER/GNS and ER/LDH. However, the simultaneous addition of GNS and LDH created an additional GNS-LDH interface, which could increase interaction in the melt and inhibit the flammable drips of ER and thus limit the flame propagation during combustion. As a result, a synergistic effect of simultaneous addition of GNS and LDH to improve the flame retardancy of ER was realized. The LOI value of ER was increased from 15.9 to 23.6 by adding 0.5 wt% of GNS and 0.5 wt% of LDH. In comparison, the LOI value was 19.5 and 21.7 when GNS or LDH was added separately, at the same level, 1 wt%. Furthermore, both GNS and LDH can increase the thermal stability of ER; there was a synergistic effect between them to decrease the THR of ER from 33.4 to 24.6.