The flame retardancy of aromatic thermotropic liquid crystal phosphorus-containing copolyester, TLCP, on PET was investigated. The results show that the presence of TLCP promotes char formation of the substrate and enhances thermal stability of char, hence delay its decomposition. SEM pictures show that the char formed from PET/TLCP is more compact, therefore is more resistant to fire and heat than that from pure PET. Evolved gas analysis by I.R measurements indicates that TLCP would decompose to produce phosphorus containing small molecular compounds in pyrolysis process. Phosphorus- containing volatile compounds are detected in gas pyrolytical products. It is suggested that TLCP could play an important role of flame retardancy in vapor phase. TLCP could inhibit the generation of combustible volatile in the pyrolytical process of PET, and therefore prevent the fire propagation during the combustion since combustible volatile is necessary for the generation of fire.