Flame retardant nanocomposites were synthesized by melt-blending phosphorus-containing copolymer (PET-co-DDP) with multi-wall carbon nanotubes. PET-co-DDP was copolymerized with terephthalic acid (TPA), ethylene glycol (EG) and 9, 10-dihydro-10[2, 3-di-(hydroxy carbonyl)propyl]-10-phosphaphenanthrene-10-oxide (DDP). The structure of the nanocomposites was characterized by scan electron microscopy (SEM). Fire property measurements by cone calorimeter revealed that the incorporation of multi wall carbon nanotubes into PET-co-DDP significantly reduced the peak heat release rates compared with the PET-co-DDP. The thermal properties of the nanocomposites were investigated by thermogravimetric analysis (TGA). It was found that the thermal stability of copolyester was improved by the addition of the multi-wall carbon nanotubes. Char formation is the main factor for these improvements. The flame retardant performance was achieved through the formation of a relatively uniform network-structured floccule layer covering the entire sample surface without any cracks or gaps. This layer re-emitted much of the incident radiation back into the gas phase from its hot surface.