For the reuse of the abundant waste rubber (WR), chlorinated waste rubber (Cl-WR) was prepared by a water-based chlorination method using chlorine gas as the chlorinating reagent. The resultant Cl-WR was used as a flame retardant of low density polyethylene (LDPE) matrix. The properties of WR and Cl-WR were characterized by thermogravimetric (TG) and Fourier transform infrared (FTIR) spectrometer. The results indicated that the Cl-WR with a 42% chlorine content had a higher char residue than WR. The limiting oxygen index (LOI) value can be increased up to 27.2 and the flame retardancy reached to the maximum (V-0) grade for the LDPE/Cl-WR-Sb blends with mass ratios of Cl-WR/LDPE and Cl-WR/Sb2O3 being 80/100 and 1.5/1, respectively. The improvement of the flame retardancy and thermal properties of the LDPE/Cl-WR-Sb blends were attributed to the char formation and synergistic effect between Cl-WR and Sb2O3 in a high temperature region, although the mechanical properties of the said blend decreased in comparison with that of the neat LDPE. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 3495-3502, 2012