Abstract : An interesting way to circumvent problems linked to use of cellulose ( high hydrophilicity, low compatibility with non-polar polymers is esterification. In this work, we describe the use of a novel solvent-free chemical pathway for grafting of fatty acid chlorides on to the hydroxyls of cellulose. This reaction was applied at different scales: from fibre to whiskers. In a first step, the process was studied on the hydrophobization of paper. The development of a modelling based on experimental results allows the estimation of the kinetics of diffusion and grafting and so a better understanding of the process. Then, the process was enlarged to other cellulose substrates with higher specific surfaces: microfibrils of wood pulp, bacterial cellulose and cotton and tunicate whiskers. The progress of the esterification was quantified by solid-state NMR and by gravimetric method. The grafting density depends not only on the nature of the substrate but also on the experimental conditions. An almost totally esterification could be achieved in specific conditions. The structural and morphological changes of the substrate at various degrees of grafting were assessed by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The characterizations by dynamic mechanical thermal analysis and by tensile tests showed that the incorporation of esterified microfibrils allows an improvement of the strength of polyethylene without a significant diminution of its tenacity.