The morphology and fracture behaviour of polycarbonate (PC)/multiwalled carbon nanotube (MWNT) composites have been studied by AFM and post-yield fracture mechanics. The essential work of fracture (EWF) method has been used to distinguish between two terms representing the resistance to crack initiation and crack propagation. A maximum in the non-essential work of fracture was observed at:2 wt.-% MWNT, demonstrating enhanced resistance to crack propagation compared to pure PC. At 4 wt.-% MWNT, a tough-to-brittle transition has been observed. The time-resolved in-situ strain field analysis revealed that the onset of crack initiation was shifted to a shorter time for nanocomposites with 4 wt-% MWNT compared to that with :2 wt.-%, and thus explained the existence of a tough-to-brittle transition in these nanocomposites.