Poly(lactic acid) (PLA)-grafted natural rubber was investigated to be used as a toughening agent for PLA. In this work, two types of reactants, which are poly(lactic acid) (PLA) and L-lactide (LA), were used for grafting with natural rubber (NR). The toughening agents were prepared through reactive melt blending by using organic peroxide as an initiator in a Brabender™ internal mixer at a rotor speed of 60 rpm, 160 °C for 12 minutes. Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR) were determined to confirm the successful grafting of PLA and LA onto natural rubber. The PLA-g-NR and LA-g-NR were then used as toughening agents for PLA by blending it with neat PLA in an internal mixer. The effects of rubber content and peroxide content on the impact property of the blends were compared to those of unmodified PLA/NR blends. Furthermore, the morphology of the blends and the phase compatibility were observed by scanning electron microscopy (SEM).