To improve the tensile strength, the bond strength, and the ablation resistant properties as well as the elongation at break of the short aramid fibers reinforced EPDM-based thermal insulation composites, two kinds of boron-containing phenolic resin, thermoplastic (TPBPR) and thermosetting (TSBPR), were added into the composites with a series of weight ratio of TPBPR and TSBPR, such as 0 : 20, 5 : 15, 10 : 10, 15 : 5, and 20 : 0. The effects of TPBPR and TSBPR weight ratio on the mechanical, bonding, and ablative properties of the composites were investigated systematically, respectively. The results showed that the crosslink density, the tensile strength, the bond strength, and the ablation resistant abilities of the composites decrease continuously with the increasing weight ratio, which confirms that the mechanical reinforcement and the ablation resistant abilities of TSBPR are higher than that of TPBPR. However, the elongation increases sharply with TPBPR and TSBPR weight ratio increasing. Therefore, the optimal weight ratio of TPBPR and TSBPR should be 10 : 10 to obtain the very thermal insulation composites with the excellent comprehensive properties. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 266-274, 2010