Microfibrillated cellulose (MFC), which consists of a web-like array of cellulose fibrils having a diameter in the range of 10-100 nm, was incorporated into a cellulose acetate (CA) matrix to form a totally biobased structural composite. Untreated and a 3-aminopropyltriethoxysilane (APS) surface treated MFC was combined with a CA matrix by film casting from an acetone suspension. The effectiveness of the surface treatment was determined by infrared spectroscopy and X-ray photoelectron spectroscopy. The Young's moduli of APS treated MFC composite films increase with increasing MFC content from 1.9 GPa for the CA to 4.1 GPa at 7.5 wt % of MFC, which is more than doubled. The tensile strength of the composite film increases to a maximum of 63.5 MPa at 2.5 wt % compared to the CA which has a value of 38 MPa. The thermal stability of composites with treated MFC is also better than the untreated MFC. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 153-161, 2010