Sodium carboxymethyl cellulose/polyethyleneimine (CMC/PEI) composites were fabricated as a function of PEI contents by a simple solution mixing and casting method. The anionic polymer CMC and the cationic polymer PEI were combined by ionic interaction. The characterization and mechanical behaviors of CMC/PEI composites were analyzed using X-ray diffraction (XRD), scanning electron microscope (SEM) and universal testing machine (UTM). Concretely, the structural properties and morphologies of CMC/PEI composites were investigated by using XRD and SEM, respectively. The mechanical behaviors of CMC/PEI composites were analyzed by UTM. It was found that tensile strength and Young’s modulus of CMC/PEI composites were slightly decreased, but failure strain was increased.