Cyanate ester (CE) resin was modified with renewable resource, i.e. epoxidized soybean oil (ESO), and the effects of ESO content on the curing co-reaction, morphologies, water absorption behaviors, thermal and mechanical properties of CE/ESO blends were studied. Differential scanning calorimeter (DSC), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and dynamic mechanical analysis (DMA) were employed to characterize the ESO-modified CE polymer networks. Homogeneous structures were observed for low-content of ESO modified CE systems by SEM, while ESO-rich particles were observed in the modified systems with ESO above 15 wt%. The blend of the CE and ESO resulted in an excellent combination as a new biobased thermoset material having relatively high mechanical properties with 15 and 20 wt% ESO as replacement of CE. Enhanced elongations at break were observed for the modified systems while the tensile strengths kept about the same level at the same time. The storage moduli and glass transition temperatures of the modified systems in the glassy state and rubber plateau were observed to be lower than those of neat CE with the increase of ESO weight percent.