A biobased monofunctional compatibilizer (called Epicard) is first synthesized by the reaction of cardanol with epichlorohydrin as confirmed by Fourier transform infrared and H-1 NMR Subsequently, biosourced polymers, polylactide (PLA) and starch, are melt-blended by a twin-screw extruder with Epicard. Confirmed by the measurements of contact angle and H-1 nuclear magnetic resonance of the extracted starches from PLA/starch/Epicard blends, the Epicard only possesses a monoepoxy group to mainly react with starch and then increased the hydrophobicity of the starches during the melt-blending process. As a result, an obvious improvement to the interfacial adhesion between starch and PLA is observed by the scanning electron microscopy. Furthermore, the tensile properties of PLA/starch blends are effectively improved with the addition of Epicard. This study suggests a simple but effective material technique by utilizing a novel plant oil modifier to increase interfacial adhesion in fabricating fully biobased PLA/starch blends with superior tensile properties.