Immobilizing poly(butyl acrylate) (PBA) on cellulose microfibrils (CMFS) by atom transfer radical polymerization, (ATRP)of butyl acrylate (BA) on the surface of 2-bromoisobutyryl-fundionalfzed CMF generated highly hydrophobic microfibrils (CMF-PBA) with a hard core and a softshell structure. TGA and staic water contact angle results suggested that the surfaces of the Modified CMF samples were not completely covered by PBA chains until the molecular weight of grafts became sufficiently long. The GPC results indicated that the grafts with low molecular weight showed controlled/"living" characteristics of the surface initiated ATRP; however, there existed more side reactions with the increase in molecular weights. Biocomposites consisting of polypropylene (PP) and CMF-PBA samples exhibited significantly improved compatibility, interface adhesion, and mechanical properties with the increase in PBA graft length. The findings confirmed that the longer grafts facilitated the better entanglement of PBA grafts with PP macromolecules and thus further improved the mechanical properties.