The effect of crystallization rate on the epitaxial interface layer thickness of high-density polyethylene (HDPE) in the epitaxial system with oriented isotactic polypropylene (iPP) has been investigated by electron microscopy. The results of bright-field electron microscopy and electron diffraction indicate that the epitaxial growth of HDPE on iPP takes place only on the interface between them. The thickness of the epitaxial layer of HDPE is markedly affected by the crystallization rate of HDPE. The critical layer thickness of epitaxial HDPE is about 100 nm under slow crystallization conditions, e.g. isothermal crystallization at 124 degrees C. When the crystallization rate is higher (quenching into air at room temperature), the epitaxial layer thickness of HDPE increases up to 250 nm.