The microstructural and mechanical properties of isotactic polypropylene (iPP), in situ PET microfibrils, and β-nucleating agent blends obtained from micro-injection molding were investigated via polarized light microscopy, differential scanning calorimetry, scanning electron microscopy, and two-dimensional wide-angle X-ray diffraction. The results indicate that addition of PET microfibrils markedly increases crystallization temperatures, and increases the thickness of the final oriented layer. Introduction of PET microfibrils to β-nucleation agent-nucleated iPP samples leads to formation of oriented β-crystals epiphytic on the surface of PET fibers in the inner region; this feature improves adhesion between the fiber and the matrix and simultaneously improves the strength and toughness of the final PP/0.5/15 microparts (e.g., the tensile strength increased by 12 MPa and the elongation at break increased by 1.2%) compared with those of iPP microparts. Taken together, the results of this study introduce an alternative approach to optimize the properties of MIM parts.