Abstract: The flame retardant melamine cyanurate (MCA) was added into glass fiber reinforced nylon 66 composites (GF-PA66). The UL-94 test was used to investigate the flame retardant effect of MCA on GF-PA66 composites. The morphology of burnt samples was observed via Scanning Electronic Microscopy (SEM) and Thermal Gravimetric(TG) analysis was used to observe the thermal decomposition behaviors of GF-PA66. The UL-94 results demonstrated that the flame retardant effect can reach UL-94 V-0 rate. The SEM and TG results showed that MCA dispersed homogenously in nylon matrix, which could adsorb heat and release noncombustible gas to isolate the samples from the air, and thus prevented fire from spreading and left nano-size pores on burning surface.