Abstract: The research mainly focuses on polymer nanocomposite formation and its thermal and flame properties. In chapter I, introduction, a brief review of the traditional flame retardants and the evaluation of the flame properties is introduced. As a new generation of flame retardants, clay has attracted much attention from researchers around the world. The clay structure and its modification are also introduced and nanocomposite formation and its characterization are discussed. The general experimental information, including materials and instrumentation, is listed in chapter II. In chapter III, polyethylene nanocomposites are fabricated based on organically-modified clays. Different surfactants, with or without functional group, are examined. All the nanocomposites show the same reduction in peak heat released rate (PHRR) as measured by cone calorimetry. In order to improve the nano-dispersion, one of the organically-modified clays was chosen to be further modified by covalent bond formation based on the reaction between edge hydroxyl groups and chlorotrimethylsilane. The nano-dispersion of the newly modified clay in polyethylene matrix is improved but with only a small reduction of PHRR, which probably means that the edge hydroxyl groups play an important role. In chapter IV, a number of oligomeric surfactants are synthesized and cation exchanged with clay. The oligomerically-modified clays are melt blended with polyolefins and styrenic polymers to fabricate polymer/clay nanocomposites. The thermal stabilities and flame properties of the nanocomposites are evaluated, in which the nanocomposites based on triclay show the best reduction in PHRR with all polymers. In chapter V, the synergistic effect is examined by combining traditional flame retardants, metal hydroxides, with oligomerically-modified clays. The presence of oligomerically-modified clay can significantly reduce the amount of metal hydroxide while maintaining the flame properties as measured by cone calorimetry. Finally, a thermally stable carbocation is synthesized and cation exchanged with clay. Polystyrene nanocomposite is successfully achieved by emulsion polymerization and results in a significant reduction in PHRR.