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Microfibrillated cellulose (MFC) is nano-sized cellulose fibers which have high molecular weight and high crystallinity that can be used as reinforcement and filler in composite polylactic acid (PLA) composites. Incompatibilities in properties between the two polymers will decrease the mechanical properties of the PLA composites. Therefore, attempt to improve mechanical and physical properties was done by modifying the MFC to MFC-acetate. Degree of substitution (DS) of the MFC-acetate tested in these PLA composites were 0.2, 0.5, and 0.8. The success of the substitution acetyl group was evidenced by infrared absorptions, i.e. reduced OH group at 3300 cm-1, existence of C=O stretching at 1770 cm-1 and C-O acetyl at 1235 cm-1. Prior to preparing the composites, the MFC was reduced its water content using solvent inclusion with ethanol, acetone, and dichloromethane, respectively. The composites were prepared by mixing PLA (molecular weight of 200,000) and the modified MFC at 9:1 ratio. Followed by 8 minute-kneading and hot pressing at 180 C, thermal and mechanical properties of the samples were tested. Compatibility and dispersibility in the composites were achieved by increasing the DS value. MFC-acetate modification was able to increase the thermal and mechanical properties of the PLA composites. The best thermal and mechanical properties was indicated by DS 0.5, with high heat resistance, low crystalline point and low melting point, which were 88.15 C and 166.41 C, respectively. The mechanical properties of the resulting DS 0.5 showed the highest strain, tensile strength, and modulus of elasticity, which were 1.55 persen, 31.29 MPa, and 2.74 GPa, as compared to pure PLA and PLA/MFC composites.