Thermoplastic nylon powder was added to naphthalene epoxy to serve as a stress release agent to reduce the stress resulting from the shrinkage during the cure of naphthalene epoxy. The purpose of this study was to explore the physical impact and effect on the forming object after adding nylon powder onto naphthalene epoxy. Mechanical properties were explored through the Izod impact test, the three-point bending test, tensile test, and lap shear adhesion test. Thermal mechanical analysis (TMA) and dynamic mechanical analysis (DMA) were conducted to identify the coefficient of thermal expansion (CTE) and the glass transition temperature (T-g). The rate of water absorption was measured via a test of pressure cook test (PCT), and insulation resistance was assessed through the breakdown voltage experiment. The results indicate that the addition of nylon powder increases the fracture energy of the cured epoxy; however, mechanical properties (lap shear strength, flexural strength, tensile strength) decreased slightly. The TMA and DMA results showed that the CTE (alpha(1)) decreased when nylon was added and the heat resistance decreased a little. The water absorption rate test and PCT showed that the rate of water absorption increased to a small extent, whereas the breakdown voltage decreased slightly. (c) 2006 Wiley Periodicals, Inc.