Multi-walled carbon nanotubes (MWNT) and tungsten disulphide nanotubes (WS2-INT) have been widely used to improve the strength and toughness of composite materials. The mechanisms of such improvements are extensively studied, but it is not often clear what prompts a specific reinforcement mechanism to work. In this work we prepared two similar systems reinforced with different nanofillers (MWNT and WS2-INT). Using in situ optical microscopy and post-fracture electron microscopy, we established that using different nanofillers results in a different type of fracture and a different reinforcement mechanism. When compared to non-reinforced composites both systems showed significant improvements in both strength and fracture toughness.