Cellulose membranes and cellulose/casein blend membranes were successfully prepared from a new solvent system (6 wt % NaOH/4 wt Q urea aqueous solution) by coagulation with a sulfuric acid aqueous solution. The structures and properties of the membranes were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), wide-angle X-ray diffraction, differential scanning calorimetry, and a tensile test. The experimental results showed that the suitable coagulation condition was 5 wt % H2SO4 for 5 min. When the cast;in content of the mixture was less than 15 wt %, the blend membranes were miscible because of the interactions between the hydroxyl groups of cellulose and the peptide bonds of casein. The blend membranes with 10 wt % casein had good miscibility, higher crystallinity, and the highest mechanical properties and thermal stability. In this case, the tensile strength and breaking elongation of the blend membranes were 109 MPa and 16%, respectively, and its pore size, obtained by SEM, was 290 nm, which suggests that the blend membranes provide a potential application for the field of separation technology.