The thermal and mechanical properties and water absorption of sodium dodecyl sulfate (SDS)-modified 11S soy protein and molded plastics made from it were studied using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), mechanical tests, and scanning electron microscopy (SEM). The DSC results showed that both the temperature and enthalpy of thermal denaturation of modified 11S solutions decreased as the SDS concentration increased. Nonfreezing water of the modified 11S solution had a minimum value at 1.0% SDS, The ordered structure of SDS-modified 11S protein was recovered and/or newly formed during the freeze-drying process. Both DSC and DMA results showed that SDS was a plasticizer of 11S,,and the glass transition temperature of modified 11S plastics decreased with increasing SDS concentration, Both the tensile strength and elongation of modified 11S plastics first decreased and then increased as the SDS concentration increased, and 5.0% SDS-modified 11S plastic had the highest tensile strength and elongation. The SEM observations supported these results. A water-absorption test showed a reduction in the water resistance of 11S plastics after SDS modification. The rate of water absorption increased with increasing SDS concentration. The hydrophobic interaction between SDS molecules and 11S protein was found to have important effects on the thermal and mechanical properties and the water absorption of 11S plastics,