There is a growing interest in the use of cellulose fibers as the reinforcements for green polymer composites. Biodegradable composites of poly(lactic acid) (PLA) incorporating cellulose nanowhiskers (CNWs) have been investigated with the goal of improving physical properties. The aim of this study was to prepare and characterize the novel homogeneous PLA-CNWs composite using whole milk casein protein as a binder. Casein is able to bind to PLA perhaps through hydrophobic interaction. Casein functionalized CNWs were mixed with PLA in dimethylformamide in order to get well dispersed suspension. The mixture was solvent casted on a Teflon plate and hot pressed to get homogenized sheets. CNWs without casein functionalization showed aggregation in the composite under electron microscope observation. The dispersion of CNWs was more homogeneous in the presence of casein. This indicates casein was able to improve the distribution of the CNWs in the matrix. The mechanical behaviors of the composites have been studied. Preliminary results indicate no further increase of modulus and strength of the novel composite perhaps due to relative low loading of CNWs.