The effect of oxygen plasma treatment of fiber on the adhesion between regenerated cellulose fiber and polyethylene (PE) was investigated using the single-fiber fragmentation test. In addition to allowing the calculation of the interfacial shear strength, the fragmentation test gave extensive information on shear transfer and failure mechanisms in the systems. It was found that oxygen plasma treatment enhanced the interfacial adhesion considerably, as established both by the calculated shear strength values and the birefringence patterns observed. The influence of treatment time on the adhesion was studied and fond to be a most important parameter. The role of surface chemistry and surface energetics of fiber in the interaction balance was investigated using electron spectroscopy for chemical analysis (ESCA) and contact angle measurements. It was seen that the changes in surface energetics owing to treatment had no positive effect on adhesion. Instead, the improved adhesion was ascribed to the presence of hydroperoxides, as detected by ESCA, on treated fibers. Decomposition of these peroxides is believed to occur at the production of single fiber specimens, and covalent bonds are formed between fiber and matrix.