By using resin transfer molding (RTM) technology, carbon fiber (CF) reinforced epoxy composites (CF/Epoxy, CF/PES/Epoxy, and CF/PES-MWCNT/Epoxy) are prepared. CF is the reinforced fiber, Epoxy is the epoxy matrix, and PES-MWCNT is the multiwall carbon nanotubes doped polyethersulfone film. Their fire behavior is systematically studied according to limiting oxygen index (LOI), vertical burning test (UL94), and cone calorimeter test (CCT). Their thermal degradation process, kinetics, and flame retardant mechanism are investigated from thermogravimetric analysis (TGA) and scanning electronic microscopy (SEM). The results show that these composites can reach the highest UL94 rating with the 5VA level achieved. Moreover, introduction of PES-MWCNT improves the LOI values, reduces the total smoke production (TSP), and slightly decreases peak of heat release rate (PHRR) and total heat release (THR). This improvement in the flame retardancy results from the increased degradation activation energy (E-a) and formation of stable protective char layers in the combustion process, as indicated by the thermodegradation kinetic analysis and the SEM observation of the char residues' microstructure, respectively.