A series of glycerol-plasticized soy protein plastics containing castor oil were prepared by intensive mixing and hot pressing. The microstructure and properties of the resulting plastics were characterized with scanning electron microscopy (SEM), dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), and mechanical tests. With small amount of castor oil incorporated (glycerol/oil ratio above 8:2), castor oil dispersed in the protein matrix homogeneously. At high concentrations, phase separation occurred. The adding of castor oil led to significant increase of storage modulus as well as glass transition temperatures attributed to glycerol-rich and protein-rich domains. Plastics containing castor oil exhibited improved tensile strength and Young's modulus under high humidity (75% RH), compared with neat glycerol plasticized protein plastics. Especially, incorporation of low content of castor oil (glycerol/oil ratio = 9:1) would result in the simultaneous enhancement of tensile strength, elongation at break and Young's modulus. Increasing castor oil content also enhanced the thermal stability of the protein plastics. (C) 2011 Elsevier Ltd. All rights reserved.