Composites used in construction, rail, and maritime industries frequently use polyester resin cross-linked with styrene. However, the inclusion of biofibers and volatile styrene makes polyester resin-based biocomposites and bionanocomposites susceptible to fire. Strict fire safety laws are also necessary because biocomposite components are heat-and fire-sensitive. Numerous flame-retardant treatments and methods, such as layered silicates, nanofillers, halogenated and non-halogenated flame-retardants, and chemical changes such copolymerization or grafting, have been developed to solve this problem. Green biocomposites have undergone some improvement in their heat resistance by physical and chemical treatment, but they still don’t match the strict fire safety standards. In order to improve flame retardancy techniques, find more appropriate and environmentally friendly flame-retardants, and choose suitable natural fibers and biopolymers for the development of fire-safe biocomposite products, a thorough investigation into the thermal breakdown and fire behavior of green biocomposites is required. In this chapter, the research done over the last decades on flammability of polyester biocomposites and some recent research on the reduction of flammability of biocomposites and bionanocomposites are reviewed.