Epoxidized soybean oil (ESO) containing octadecyl trimethyl ammonium functionalized montmorillonite (OMMT, 1–5 wt.%) were thermally cured using methylhexahydropthalic anhydride in the presence of 2-ethyl-4-methylimidazole catalyst. The mechanical properties of ESO nanocomposites were studied through the tensile and fracture toughness tests. The thermal properties were characterized by means of dynamic mechanical analyzer, thermogravimetric analyzer and differential scanning calorimeter. The addition of OMMT into ESO significantly improved the modulus, tensile strength, glass transition temperature, crosslink density, gel content and thermal stability of ESO/OMMT bio-thermoset. These are mainly attributed to the well-exfoliated OMMT and the co-catalytic effect of octadecyl trimethyl ammonium.