In this work, starch nanocrystals were chemically grafted with lactic acid using esterification reaction and its compatibility with poly (lactic acid) (PLA). Initially, ungrafted and grafted starch nanocrystals were characterized to understand the crystalline, functional, thermal and morphological properties by means of wide-angle X-ray scattering, Fourier transform infrared, X-ray photoelectron spectroscopy, thermogravimetric analysis and transmission electron microscopy, respectively. The results confirmed that the surface of starch nanocrystals was successfully modified with lactic acid. Subsequently, grafted starch nanocrystals were blended, in solution, with PLA at different concentrations ranging from 5 to 30wt%. Then, starch nanocrystals/PLA films were prepared using solvent casting technique. The influence of the grafted starch nanocrystals, at different concentrations, on thermal, mechanical and morphological properties of resulting PLA nanocomposites was investigated using differential scanning calorimetry, thermogravimetric analysis, dynamic mechanical analysis and scanning electron microscopy, respectively. The results revealed that the interfacial adhesion and the compatibility between starch nanocrystals and PLA matrix were substantially improved by the grafting. This improved compatibility between grafted starch nanocrystals and PLA led to a significant increase in PLA nanocomposites crystallinity as compared to neat PLA.