The flame retardancy of low-density polyethylene (LDPE) treated with complex flame retardant composed of ultrafine zinc berate (UZB) and intumescent flame retardant (IFR) have been investigated by limited oxygen index (LOI), UL-94 test, thermogravimetric analysis (TGA), cone calorimeter test, scanning electron micrograph (SEM), energy-dispersive spectrometer (EDS), and X-ray diffraction (XRD). The results of LOI and UL-94 test indicate the desired flame retardancy of LDPE is obtained when the mass ratio of UZB to IFR is 4.2: 25.8 and the complex flame retardant mass content is 30% (based on LDPE). The results of cone calorimeter show that heat release rate (HRR) peak, total heat release (THR), and mass loss of LDPE/IFR/UZB decrease substantially when compared with those of LDPE/IFR. TGA results show that the residue of LDPE/IFR/UZB increases obviously than that of LDPE/IFR when the temperature is above 600 degrees C. SEM indicates the quality of char forming of LDPE/IFR/UZB is superior to that of LDPE/IFR. The results of EDS and XRD indicate that boron orthophosphate (BPO4) and zinc-contained compounds are formed in the residual char and these substances may play an important role in stabilizing the intumescent char structure and decrease the degradation speed substantially when subjected to high temperature. (c) 2006 Wiley Periodicals, Inc.