In the aim of producing fully organic bionanocomposite based on poly(lactide) (PLA), cellulose nanowhiskers (CNW) were grafted by n-octadecyl-isocyanate (CNW-ICN) applying an in situ surface grafting method. The compatibilizing effect of the long aliphatic grafted chain was investigated by thermal, mechanical and permeability analysis of solvent cast nanocomposite films. The grafted CNW-ICN could be successfully dispersed in the polymer matrix. The gained compatibility brought about a nucleating effect, decreasing the half time of isothermal crystallization from 25 min for the neat PLA to 8.4 min for the nanocomposite including 2.5 wt% CNW-ICN, e.g., tensile strength was improved by 10 MPa for the same 2.5 wt% CNW-ICN/PLA composite. Mechanical reinforcement was also effective in the rubbery state of PLA and increased the tensile modulus of the rubbery plateau providing thereby thermal resistance to the polymer. Oxygen barrier properties did not change significantly upon the inclusion of CNW-ICN, even when the quantity of CNW-ICN was increased to 15 wt%. More interestingly, the water vapour permeability of the CNW-ICN nanocomposite was always lower than the one of ungrafted CNW composites, which led to the conclusion that the hydrophobic surface graft and improved compatibility could counteract the effect of inclusion of hydrophilic structures in the matrix on water vapour transport. In conclusion, the surface grafting of CNW with isocyanates might be an easy and versatile tool for designing fully organic bionanocomposites with tailored properties.