High-strength, high-modulus products with a characteristic phase morphology were prepared via the injection molding of blends of isotactic polypropylene (PP) and polyamide-12 (PA-12) under special processing conditions (low melt temperature, high holding pressure, and a convergent sprue design). These conditions generated a situation in which the dispersed phase can be distributed in the self-reinforced matrix as oriented fibrils. Optical (polarized light optical and phase contrast) microscopic and SEM investigations revealed a uniform distribution of the dispersed phase within the matrix. These products can be regarded as in situ thermoplastic composites. The strength level of the self-reinforced blends is approximately twice as high as that of the products processed under conventional conditions. The PA-12 phase dispersed as ultrathin fibrils enhances the flexural impact strength of the blends, decreases their notch sensitivity, and alters its fracture mechanism.