Blocked polyurethane (BPU)/phenolic (PF) interpenetrating polymer network (IPN) were prepared from BPU prepolymer with m‐xylylenediamine (MXDA) as a chain extender and PF prepolymer using p‐toluene sulfonic acid (PTSA) as a catalyst. The kinetic aspects of the formation of a second network in the presence of the first have been studied in BPU/PF IPN. The effect on kinetics of parameters such as the BPU content and the reaction temperature has been examined. The studies of swelling ratio, thermal properties, and flame retardance of IPN are based on BPU content and filler content. Results showed that the swelling ratio of IPN increased with the increasing of BPU content, and decreased with the increasing of filler (kaolin) content. The IPN possesses good thermal properties of weight loss and thermal degraded temperature by thermogravimetric analysis (TGA) test when the IPN decreased the BPU content and increased the filler (kaolin) content. The IPN showed the excellent flame retardance when the IPN decreased the BPU content. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers