Three-dimensional (3D) braided polyethylene (PE) fiber-reinforced poly(methyl methacrylate) (PMMA), denoted as PE3D/PMMA, composites were prepared. Mechanical properties including flexural and impact properties, and wear resistance were tested and compared with those of the corresponding unidirectional PE fiber-PMMA (abbreviated to PEL/PMMA) composites. Both untreated and chromic acid-treated PE fibers were used to fabricate the 3D composites in an attempt to assess the effect of chromic acid treatment on the mechanical properties of the composites. Relative changes of mechanical properties caused by fiber surface treatment were compared between the PE3D/PMMA and PEL/PMMA composites. The treated and untreated PE fibers were observed by scanning electron microscopy (SEM) and analyzed by X-ray photoelectron spectroscope (XPS). SEM observations found that micro-pits were created and that deeper and wider grooves were noted on the surfaces of the PE fibers. XPS analysis revealed that more hydroxyl (-OH) and carboxyl (-COOH) groups were formed after surface treatment. The physical and chemical changes on the surfaces of the PE fibers were responsible for the variations of the mechanical properties of the PE/PMMA composites. (c) 2005 Wiley Periodicals, Inc.