Crystalline structures in the melt penetrating process of iPP into HDPE during multi-melt multi-injection molding, in which the cavity was partially filled with the HDPE-melt, and then HDPE-melt was penetrated by the second-melt iPP, finally the cavity was completely occupied and then cooled down gradually, were investigated by polarized light microscope, scanning electron microscope and x-ray diffraction. Interestingly, along the interface between HDPE and iPP,β- cylindrites of polypropylene emerged. In fact, three common factors, nucleating agent, shear and temperature fields, can lead to the formation ofβcrystals of polypropylene. In order to exclude the effect of shear and temperature fields on the formation ofβ-polypropylene, the process of iPP penetrating iPP was also conducted, however, nothing but α-spherulites of polypropylene were found in all the layers of the specimen. Therefore, the results showed that the HDPE may act as someβ-nucleating effect on the formation of β-polypropylene in the process of iPP-melt penetrating HDPE-melt. It was also found that the banded spherulites were formed in the skin-layer by PLM observation.