Recycled poly(ethylene terephthalate) from waste bottles (hereafter, rPET) was used as an reinforcing material for isotactic polypropylene (iPP) based on the concept of in situ microfibrillar-reinforced composites (iMFCs). Microfibers of rPET were successfully generated during melt-extrusion and subsequent drawing and preserved in the final injection-molded specimens. The effects of draw ratio, initial size of ground rPET flakes, and rPET content on morphological appearance of the extrudates and the as-formed rPET fibers and mechanical properties of the as-prepared iMFCs were investigated. The results showed that diameters of the as-formed rPET fibers decreased with increasing draw ratio, and the initial size of ground rPET flakes did not affect the final diameters of the as-formed rPET fibers nor the mechanical properties of the as-prepared iMFCs. Flexural modulus, tensile modulus, and tensile strength of iPP/rPET iMFCs were improved by the presence of rPET microfibers and further improvement could be achieved by the addition of maleic anhydride-grafted iPP (PP-g-MA), which was used as the compatibilizer. (c) 2006 Wiley Periodicals, Inc.