The influence of aluminum hypophosphide (AlHPi) and nanosilica on the flame-retardant and mechanical property of ethylenepropylenediene monomer (EPDM) rubber was evaluated by limiting oxygen index, and the value of tensile strength and elongation at break. The results show that the introduction of nanosilica into the EPDM/AlHPi blends can not only further improve the flame-retardant property but also improve the tensile strength and elongation at break significantly, showing a synergistic effect between AlHPi and nanosilica. The flame-retardant mechanism was further studied by X-ray diffraction (XRD), thermogravimetric (TG) analysis, differential scanning calorimetry (DSC), and scanning electron microscope (SEM). The results of XRD and TGDSC indicate that AlHPi will melt along with oxidation at about 337 degrees C, which is helpful to full contact with nanosilica and to enhance the interaction between them; and will further recrystallize above 540 degrees C, which is benefited to enhance the mechanical strength of char layer. The char morphological study by SEM shows that the char layer for the sample with both AlHPi and nanosilica is strong, more uniform and dense, and the scale of the holes in the char layer is smaller compared with the char layer of samples with AlHPi or nanosilica alone. The TGDSC results show that the sample with both AlHPi and nanosilica has the weakest weight loss rate and heat release rate, compared with the samples with either of them, which is another evidence of the synergistic flame retardant effect between AlHPi and nanosilica. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012