Alginates and chitosan are natural biopolymers with desirable biological properties. Sodium alginate/poly(ethylene oxide) (PEO) and chitosan/PEO blend films that are non-toxic, biodegradable, and biocompatible were prepared using the solvent casting method. The characteristics of the blend films with different blending ratios were investigated using a universal testing machine, differential scanning calorimeter, Fourier transform infrared spectrometer, contact angle apparatus (goniometer), and swelling test. The effect of the addition of a plasticizer on the swelling properties was evaluated for the chitosan/PEO blend films. With decreasing PEO content, the tensile strength of the blend film increased, but its strain decreased. The compatibility between sodium alginate or chitosan and PEO was confirmed, and the swelling properties could be controlled by the addition of a hydrophilic plasticizer. Therefore, the blending of natural polymers (with certain biological properties) with a synthetic polymer (PEO) could be a promising method to prepare biomedical materials with controllable properties.