From food packaging to electronics, plastics are used every day around the world. But with the reliance on plastic materials, comes the concern about their poor biodegradability. The effort to develop more 'green' materials continues to gain significant momentum. Soybeans are abundant in the United States, making up about one third of crops planted by area in the U.S. in 2017. Up to 38 wt % of soybeans is protein content, which has been considered as one promising 'green' material source. However, the knowledge of the functional properties of soy protein in material applications is still very limited. This study analyzed the effects of soy protein isolate (SPI) as a rheological modifier to a common plastic, polyethylene oxide (PEO). Two groups of PEO/SPI composites were made by using deionized water and dimethyl sulfoxide (DMSO) as the solvents, with the intention to create different SPI structures and PEO-SPI interactions. Meanwhile, the effects of concentrations of SPI and temperatures on the gelation performance of SPI in PEO have been investigated. The linear viscoelastic properties were studied by rheological analysis. It is found that the addition of a small amount of SPI was able to enhance the moduli of the PEO melt, and reduce their frequency dependence, leading to solid-like viscoelastic behaviors, in contrast to a typical liquid-like behavior of pure PEO. Moreover, PEO/SPI-DMSO composites performed more like solids than PEO/SPI-H2O composites. SPI concentration also played a role in the viscoelastic behavior, with higher concentration resulting in higher elasticity of the composite melt.