Nanofillers such as carbon nanotubes and clay are attractive materials, because addition of small amount of nanofillers can improve mechanical, thermal and electrical properties of plastics without changing processability. However, nanofillers themselves do not show excellent fire retardancy such as self-extinguish properties. Nanofillers should be combined with other fire retardants. Some combination showed positive synergy effect in fire retardancy, but some case showed negative synergy. It is important to know fire retardant mechanism of nanofiller to develop more efficient fire-retardant nanocomposites. In this chapter, we’ll show the fire retardant mechanism of nanofillers. Then, effective combination of nanofiller and conventional fire retardant is introduced reviewing lots of papers.