Natural fiber reinforced poly(lactic acid) (PLA) composites have received great attention. However, the poor interfacial adhesion between strong polar natural fibers and the non-polar PLA matrix limited its applications. In the present study, rice straw fiber (RSF) was pre-treated to improve its compatibility with PLA. Methyl methacrylate (MMA) was selected as the monomer in the admicellar polymerization for the RSF treatment, because RSF coated with poly(methyl methacrylate) (PMMA) thin film would have better compatibility with the PLA matrix according to a preliminary estimated using Hansen solubility parameters. Fourier transform infrared spectroscopy (FUR) and scanning electron microscopy (SEM) indicated the evidence that PMMA film was coated on RSF. Then, the influences of treatment on electrophoretic properties and crystallinity of admicellar-treated RSF (TRSF) were examined by zeta (zeta) potential and X-ray diffraction (XRD) measurements, respectively. The (T)RSF-PLA composites were prepared by injection molding machine, the mechanical properties showed that the tensile strength of TRSF-PLA increased significantly at all TRSF contents (10, 20, 30% v/v). The elongation of TRSF-PLA (0.1%, v/v MMA) composites was increased 60.8% compared with RSF-PLA composite (30%, v/v fiber content). The significantly improved mechanical properties were attributed to the interfacial adhesion improvement between TRSF and PLA which was investigated by SEM. The thermal properties of (T)RSF-PLA were investigated by thermogravimetric analysis (TGA), and the TGA results confirmed that the thermal stability of TRSF-PLA was improved compared with RSF-PLA composite. (C) 2011 Elsevier B.V. All rights reserved.