A synergistic effect on flame retardancy was found when acrylonitrile butadiene ultra-fine fully vulcanized powdered rubber (NB-UFPR) was incorporated into ethylene vinyl acetate/nano-magnesium hydroxide (EVA/nano-MH) composite by a new process. The fire performance of EVA and EVA composites was compared in this communication by cone calorimeter test (CCT). The CCT data indicated that the addition of NB-UFPR in EVA/nano-MH system not only reduced the heat release rate, but also prolonged the ignition time of the composite, which is contrary to the effect of NB-UFPR when it was added alone in EVA polymer. Thermogravimetric analysis revealed that nano-MH accelerated the loss of acetic acid, but NB-UFPR assisted to reduce nano-MH's accelerating effect. FTIR spectra showed a new absorption at 3374 cm −1 in the blends of EVA/NB-UFPR and EVA/NB-UFPR/nano-MH.