Blocked polyurethane (PU)/epoxy full-interpenetrating polymer network (full-IPN) were synthesized from blocked NCO-terminated PU prepolymer, with 4,4-methylene diamine as a chain extender and epoxy prepolymer, with 4,4-methylene diamine as a curing agent, using simultaneous polymerization (SIN) method. From FTIR spectra analysis it was found that the major reactions in the blocked PU/epoxy IPN system are the self-polymerization of block PU/chain extender and the self-polymerization of epoxy/curing agent. Meanwhile, from reaction mechanisms the copolymerization of IPN may have occurred at the same time. The weight loss by thermogravimetric analysis decreased with increasing epoxy and filler content. It was confirmed from scanning electron micrography (SEM) that when the blocked PU content increased, the microstructure of IPN became rougher. (c) 2006 Wiley Periodicals, Inc.