ing flame retardant plastics, such as high impact polystyrene. Antimony trioxide is often used as a synergistic agent for halogenated flame-retardants. Aluminium trihydrate (ATH), also is a primitive additive for making flame retardant plastics but no report exists on synergistic effect of this compound in presence of brominated flame-retardant. In this research, the synergistic effect of Sb2O3 and ATH, in presence of tetrabromobisphenol-A as a flame-retardant agent has been compared. Limited oxygen indexes (LOI) of the compounded samples were measured by flammability tests. Also thermal gravimetry analysis (TGA) and scanning electron microscopy (SEM) were applied in order to study the thermal behaviour and morphology of the samples, respectively. Ali-Reza Mahdavian*1, Jalil Morshedian2, and Mahdieh Rezaie2