Abstract: The PP/PET in-situ fiberized composites with reinforcing effect were prepared by extrusion-drawing-injection molding. The influences of PET mass content (C_m) on the PET fiberization, tensile strength (σ_t) and Young's modulus (E) of the composites, together with their functional mechanisms were studied in contrast to the normally blended materials without drawing. The results show that as C_m rises from 0 to 20%, the number of PET fibers in-situ formed increases whereas their diameter and dispersity decrease till C_m=15% and then increase.Correspondingly, the σ_t and E of the composites increase till C_m=15% and then decrease, with maximum gains of σ_t and E of about 20% and 70% relative to neat PP,respectively. The fiberization of PET droplets through coalescence-deformation during melt-drawing plays a key role in the above variation of PET phase morphology with C_m. The competition between the reinforcing effect of dispersed phase on matrix and the interfacial flaw effect of the two phases, and the rigidizing effect of fiber on matrix controlled by both fiber number and fiber fineness, are dominant factors of deciding the above σ_t~C_m and E~C_m relations, respectively.