In this work, flame-retardant ethylene propylene diene terpolymer (EPDM) composites were prepared by incorporating intumescent flame-retardant (IFR) together with two different organically modified layered nanoparticles: montmorillonite (OMMT) and layered double hydroxides (OLDH). The morphology, fire behavior, thermal stability, flame-retardant synergism, and mechanical properties of the flame-retardant EPDM nanocomposites were studied. The introduction of a certain amount of OMMT (or OLDH) in the intumescent flame-retardant EPDM led to the considerable enhancement of flame retardance, thermal stability, and mechanical properties. Especially, the EPDM-IFR30-OLDH2. composite (100 phr EPDM, 30 phr IFR, and 2 phr OLDH) presented the highest tensile strength and could pass a UL-94 test V-0 rating; while the EPDM-IFR30-OMMT2 composite (100 phr EPDM, 30 phr IFR, and 2 phr OMMT)showed the lowest peak heat release rate (pHRR) and total heat release (THR) values. Different flame-retardant performances should be attributed to their own characteristics, dispersion state in EPDM matrix, and the change of structure during burning. The flame-retardant mechanisms for the composites were proposed to be condensed phase activity: