In the last years, the use of cellulose as a component in polymeric nanocomposites has been extensively studied [1]. Cellulose is claimed for being used as nanofiller in different kinds of polymer matrices to offer advantages such as vast abundance, renewable and environmentally benign nature biodegradability, biocompatibility, low cost, among others [2]. Furthermore, the addition of cellulose nanofillers in polymers promotes a better performance of the nanocomposite in relation to properties such as barrier, thermal and mechanical behavior [3]. The nanocellulose have been used as a reinforcing material in many natural and synthetic polymers. The aim of this work was to produce nanocomposites based on poly(lactic acid) reinforced with unmodified (CNC) or modified (M-CNC) cellulose nanocrystals by admicellar polymerization by extrusion molding process. The produced nanocimposites were characteried by FTIR, TGA, X-ray diffractometry, and measurement of mechanical properties. In this study, the admicellar polymerization was carried out using cetylpyridinium chloride (CPC) as the surfactant and MMA as a monomer in order to form a PMMA film for improvement of the interfacial adhesion nanofiller-matrix. PMMA was selected as the organic film for cellulose nanocrystals treatment because of its high physical affinity with PLA. The results of FTIR analysis of CNC and M-CNC indicated the evidence that PMMA film was coated on CNC. By XRD, was possible to observe that both CNC and M-CNC acted as nucleating agents for the crystallization of PLA and an increase in the cellulosic filler content increased the crystallinity of the materials. This effect was more pronounced when M-CNC was added on PLA. An improvement in the mechanical performance was observed with M-CNC addition. The improved mechanical properties were attributed to the interfacial adhesion improvement between M-CNC and PLA due to the treatment of cellulose nanocrystals surface by admicellar polymerization.