A novel epoxidized hyperbranched poly(phenylene oxide) (EHPPO) is designed and synthesized successfully. The structure of EHPPO is characterized by Fourier transform infrared spectra- and quantitative C-13 nuclear magnetic resonance spectrum. The synthesized EHPPO is added into diglycidyl ether of bisphenol A as a modifier in different ratios to form hybrids and cured by an anhydride curing agent. Effects of EHPPO addition on the properties of the cured hybrids are investigated. Thermal mechanical analysis results suggest that addition of EHPPO can increase the free volume of the cured hybrid materials. Dynamic mechanical analysis characterizations show that the crosslinking density increases with the increase in EHPPO content. Furthermore, addition of EHPPO results in an improvement in thermal and mechanical properties. The toughening mechanism is also discussed. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 128: 907-914, 2013