Effects of the addition of poly(ethylene oxide) (PEO) on the tensile properties of a polypropylene (PP)/fibrous cellulose (FC) composite were studied. PEO was incompatible with the PP matrix, and a PP/PEO blend showed a sea-island morphology. However, the existence of the PEO phase hardly impaired the ductility of PP, leading to a strain constraint relaxation resulting from void formation in the phase. The tensile behavior of PP/PEO was little affected by the content (until 10 wt %) or molecular weight of PEO. The results suggested that the PEO phase was able to be deformed in a slit-like shape and had no interaction with the PP matrix. Effects of PEO on the morphology and tensile and fracture behavior of the PP/FC composite with maleated polypropylene (MAPP) as a compatibilizer critically depended on the preparation method. In the case of the addition of PEO to PP/FC/MAPP, increases in the strain and fracture energy were observed in comparison with PP/FC. In the case of the addition of FC/PEO to PP/MAPP, although the obtained composite showed a lower Young's modulus and tensile strength in comparison with PP/FC, the strain and fracture energy were considerably increased by the existence of the PEO layer coating the FC. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 33623370, 2009