Aluminum hydroxide (ATH), a non-toxic and environmentally friendly flame retardant, was characterized using thermogra-vimetric analysis (TGA) and differential thermal analysis (DTA). The effect of ATH content on flammability properties of LDPE was also determined using limiting oxygen index (LOI) and smoke density test. The thermal characterization studies showed that the effective temperature range for ATH as a flame retardant is 200 to 350°C. In water release test, it was determined that the release of the water by ATH degradation in ATH increased with increasing temperature. The LOI test showed that the flame retardancy of LDPE increased with increasing ATH content and decreasing ATH particle size. The smoke density test proved that addition of ATH reduced effectively the smoke density of a burning LDPE. It was studied in this paper the effect of ATH particle size on thermal, flammability and viscosity of ATH filled with LDPE. © 2013 Carl Hanser Verlag, Munich, Germany.