Cellulose acetate (CA) and polyethylenimine (PEI) composite nanofibers were developed via the electrospinning technique. The influence of PEI concentration on the size and morphology of composite fibers was observed by SEM. The secondary fibers produced along with the primary fibers, upon the addition of PEI component. This was named as "bimodal" size distribution. To remove the remaining PEI, the as-spun membranes were immersed in deionized water. The results showed that the existence of PEI in the water-treated process promoted the swelling of fibers. When PEI content was lower, the compatibility of CA and PEI blends was enhanced by the interaction of the polar group between the NH group in PEI and the OH group in CA. The hollow structure of the composite fibers could be observed after water treating, because of the dissolution of PEI inside the fibers. With the increase of PEI content, the side polar groups showed the enhanced repulsion under the effect of the electrical force, which led to PEI diffusing outwardly to the composite fiber surface.