We developed an epoxy thermosetting system based on a reaction of an epoxy resin and epsilon-caprolactam (epsilon-CL) in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a catalyst. Ring-opening copolymerization of epoxide groups and epsilon-CL proceeded at 170 degrees C, affording gel products in high yields. The physical properties of the products were dependent on the feed ratios of the epoxy resin and epsilon-CL.