9, 10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) is a kind of cyclic phosphate, which possesses high thermal stability, oxidation resistance and excellent water resistance.DOPO and its derivatives have been widely incorporated into polymers for improving the flame retardancy and thermal stability [1,2].In this work, a novel reactive phosphorus-containing flame retardant, 9, 10-dihydro-9-oxa-1 0-phosphaphenanthrene-10-oxide maleic acid (DOPO-MA), was synthesized from DOPO and maleic acid (MA) [3].The chemical structure was confirmed by FTIR, 1H-NMR and 31p-NMR.DOPO-MA was melt blended with poly (butylene terephthalate) (PBT) to prepare flame retardant PBT/DOPO-MA composites.Thermal degradation behavior and flame retardancy of the PBT/DOPO-MA systems were investigated by the thermogravimetric analysis (TGA), the limiting oxygen index (LOI) test and UL-94 test.The results showed that the incorporation of DOPO-MA could improve the thermal stability and flame retardancy efficiently.When the content of DOPO-MA reached 20 wt%, the PBT system met the UL-94 V0 classification.As shown in Figure 1, this DOPO-based flame retardant could improved the char yield of the composite materials.Moreover, the elevating of torque during melting blending indicated that DOPO-MA might act as both flame retardant and chain extender for PBT matrix, which was beneficial to both the flame retardancy and mechanical properties.