Isotactic poly(4-methyl-1-pentene) (PMP) is a semicrystalline polyolefin with many unique properties. The crystallization theory of pure PMP had been well studied based on its molecular structure. In this study, the crystallization behavior and compatibility of PMP/polypropylene (PP) blends were investigated by differential scanning calorimetry, x-ray diffraction, scanning electron microscopy (SEM), and mechanical testing. It was found that the PP crystallization behavior was strongly affected by the presence of PMP in the blend: first, PMP crystallized and then acted as nucleating agent during the crystallization process of PP at lower temperature. In contrast, the influence of PP on the crystallization of PMP was relatively small due to the wide disparity of the melting temperatures of PMP and PP. The fracture morphology observed by SEM showed that there was no obvious phase separation structure in the blends; but the mechanical properties of PP/PMP blends are not good as expected for an ideal homogenous mixture.