AbstractThe elastic and yield properties of drawn poly(methyl methacrylate) (PMMA) filled with ultrafine SiO2are described as functions of filler content and size. The drawn PMMA composites were made by uniaxially drawing to x4.0 at 100°C and at a rate of 20 mm/min. Four compliance values, i.e.,S33,S11,S13, andS44were determined. These values decreased with filler content and decreasing filler size. The relative compliance valuesSde/Sdo(Sdeis the compliance of drawn PMMA composites andSdois that of drawn unfilled PMMA) are almost equivalently changed with changes in filler content. The elastic properties of drawn PMMA composites are thus reinforced isotopically. This is characteristic of PMMA which has a large side group. The yield behavior of drawn PMMA composites have similar filler size and content dependence to those of elastic properties except that the transverse yield stresses become more brittle with filler content. The anisotropy in yield stress is relatively larger than that of elastic properties. This is probably because the anti‐reiforcing effect, such as fibrillation becomes prominent with increasing filler content in the perpendicular directi