Epoxy acrylate (EA)/glycidyl-polyhedral oligomeric silsesquioxane (G-POSS) nanocomposites were synthesized via in situ ultraviolet initiated polymerization. XRD analysis indicates that G-POSS and EA are miscible and can form uniform composites. SEM micrographs show that the G-POSS particles (<500 nm in diameter) disperse uniformly in the polymer matrix. The EA/G-POSS nanocomposites exhibit heterogeneous morphology. FTIR analysis confirms the curing reaction is quite complete, and there are no chemical reactions between G-POSS and EA during the UV-curing process. The carbon–carbon double-bond conversion vs time profiles confirm that the addition of G-POSS improves the UV-curing rates of nanocomposites. The glass transition temperature (Tg) of nanocomposites were obtained by DMA. Tg reaches to the maximum at the loading of 1 wt% and then decreases with the increasing G-POSS loadings. The thermal stability, impact resistance, and flexibility of nanocomposites are all enhanced by the incorporation of G-POSS.