Scanning electron microscopy (SEM) was employed to investigate crack initiation and propagation process in notched and unnotched Izod impact fracture surfaces of the cellulose nanofiber (CNF) and microfibrillated cellulose (MFC)-filled polypropylene (PP) composites compared with microcrystalline cellulose (MCC)-filled composites. CNF is in the form of short fibers 50–300 nm in diameter and 6–8 in aspect ratio, MFC is in the form of long fibrils 50–500 nm in diameter and 8,000–80,000 in aspect ratio, and MCC is in the form of particles 50 μm in average diameter and 1–2 in aspect ratio. The reinforcement material size of CNF and MFC are smaller than that of MCC which means that the larger interfacial area between filler and matrix leading to larger energy dissipation at the interface during the impact fracture. The reinforcement-matrix debondings nearby MCC particles caused easy crack propagation which contributes smaller energy dissipation at the interface. A slip-stick behavior and stress whitened area during the fracture were observed. Morphological investigation helps to explain impact fracture behavior. According to essential work of fracture (EWF) analysis of Izod impact results, EWF method is applicable to analyze impact fracture behavior and the energy consumed in crack initiation and propagation during the fracture process can be calculated. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013