Dicyclopentadiene phenol epoxy resin (DCPD) was modified by polyhedral oligomeric silsesquioxane bearing benzoxazinyl groups (BZPOSS), and the corresponding nanocomposites were prepared. BZPOSS worked as co-polymer and curing agent, thus the curing reaction of DCPD and BZPOSS could undergo readily without extra curing agents. SEM images showed that BZPOSS was monodisperse in the nanocomposites with the size of around 50 nm. Dielectric properties were improved by incorporation of micro-porous BZPOSS and the lowest dielectric constant of 2.14 was obtained for poly(DCPD/BZPOSS) with 20 wt% BZPOSS. The T-g obtained by DMA tests increased with increasing amount of BZPOSS and the highest T-g was 259 degrees C, 101 degrees C higher than epoxy resin cured by 2-methylimidazole. Crosslinking density and storage modulus at rubbery state of the nanocomposites increased more than 16 times by BZPOSS. Initial decomposition temperatures of poly(DCPD/BZPOSS)s increased by more than 100 degrees C and the char yield increased about 35% with 40 wt% loading of BZPOSS.