Technical Abstract: Hydrogels formed from biopolymers or natural sources have special advantages because of their biodegradable and biocompatible properties. The viscoelastic properties of a newly developed biological hydrogel made from modified vegetable oil, and epoxidized soybean oil (ESO) were investigated. The material named HPESO is a hydrolytic product of polymerized ESO (PESO). HPESO exhibited viscoelastic solid or gel behavior above 2% (wt.-%) at room temperature and viscous liquid behavior at 55 deg C. The thermal assembly-disassembly-reassembly function of the HPESO hydrogel was completely reversible. The viscoelastic properties of HPESO were strongly dependent on concentration. The analysis of modulus, concentration dependence, and stress relaxation measurement indicated that HPESO was a physical gel meaning the cross-linking between the molecules was the physical junction. HPESO hydrogel also showed fast initial partial recovery of its viscoelastic properties after being subject to a disruption mechanical shear. The function and behavior of the HPESO hydrogel suggest that this biomaterial is a candidate for applications in drug delivery and scaffolds of bioengineering and tissue engineering.