In this work, the flammability behaviors and synergistic effects of red phosphorus masterbatch (RPM) with expandable graphite (EG) in flame-retardant high-density polyethylene/ethylene vinyl-acetate copolymer (HDPE/EVA) composites have been investigated by limiting oxygen index (LOI), UL-94 test, cone calorimeter test (CCT), thermogravimetric analysis (TGA), Fourier-transform infrared (FTIR) and scanning electron microscopy (SEM). The data obtained from LOI, UL-94 test and CCT showed that suitable amount of RPM had synergistic effects with EG in the HDPE/EVA/EG/RPM composites. The addition of RPM greatly increased the LOI values by 3.4%, obtained UL-94 V-0 rating, decreased the heat release rates and total heat release, and prolongated the ignition time when 6.7 phr RPM substituted for EG in the HDPE/EVA/EG/RPM composites. The data from TGA and FTIR spectra also indicated the synergistic effects of RPM with EG considerably enhanced the thermal degradation temperatures. The morphological observations after UL-94, CCT, and SEM images presented positive evidences that the synergistic effects took place for RPM with EG, and the flame-retardant mechanism has been changed in flameretardant HDPE/EVA/EG/RPM composites. The formation of stable and compact charred residues promoted by RPM acted as effective heat barriers and thermal insulations, which improved the flame-retardant performances and prevented the underlying polymer materials from burning. (C) 2015 Society of Plastics Engineers