This paper reported the fracture behavior of polymer-based nanocomposites reinforced by carbon nanotubes (CNTs). In order to solve the problem associated with the dispersion and re-agglomeration of CNTs, three dimensional CNT sponge was prepared and incorporated into the epoxy through the monomer infusion process. The fracture toughness and critical energy release rate of the nanocomposites were enhanced by more than 100% and 250%, respectively. The toughening mechanism behind such observation was revealed by studying the insitu crack initiation and propagation in the bulky nanocomposites with assistance of micro-mechanical testing device in a scanning electronic microscope chamber. The results showed that the intrinsic fracture behaviors, such as void generation, as well as the extrinsic toughening features arising from crack deflection and bifurcation, were responsible for the improved fracture properties of nanocomposites.